Throughout history new discoveries and detections, particularly in the areas of medicine and science, have severely challenged established thought, often pushing the boundaries of our perceptions. Many times the human race has had to re-evaluate what seemed like fixed ideas, sometimes kicking and screaming when confronted by rational truths that disputed the old, comfortable ways.

At this time, we are staring ahead at a critical point of human development. Stem cell therapies, accelerated by the facts gained from the lengthy human genome project of the 1980s and 1990s, mean that we have entered a period of massive medical, scientific and moral re-evaluation.

It is within our reach to eliminate motor neurone disease, Alzheimer's, cystic fibrosis and untold other hellish illnesses. It should be a cause for celebration. Instead we have religious figures of the fundamentalist born-again Christian movement and notably Cardinal Keith O'Brien (who elected him?), deliberately using language to conjure up images of Frankenstein's monster. What a miserable state of affairs it is when the Roman Catholic Church, having already impeded so much of this vital research, resorts to threatening excommunication of Catholic politicians who vote for stem cell therapies to be licensed and developed.

As one who suffers from motor neurone disease, I have followed the human genome project and the ground-breaking discovery of scientists being able to isolate the nucleus of the cell. In discussion with others, I notice the inability of most to differentiate between a cell, a neurone, a nucleus, an embryo, a clone and a foetus. And it is in the misuse of these terms that Cardinal O'Brien and his church manipulate language in order to exploit a lack of knowledge among the general population. On the official Roman Catholic Church's website, it is claimed that stem cell research has not yielded any worthwhile results. In fact, scientists are overwhelmingly positive and confident that these devastating illnesses can be consigned to history.

It would be a simple argument to tell Catholics or other religious people that they are indeed at liberty to refuse these treatments, much as Jehovah's Witnesses refuse blood transfusions. But that would suggest the Catholic Church has little or no influence, which is not the case. Hundreds of millions of people throughout the world hang on every word of bishops, cardinals and, of course, the Pope. This matter is so serious that we must now ask if we are to base political decisions involving health and medicine on the absurd idea that a just-fertilised cell, which has no brain, no organs or bones, no personality and no central nervous system, equals a human being.

Kenny McGuigan, Coatbridge.

What on earth is Gordon Brown thinking of? Alone among party leaders, he imposes a three-line whip on a "conscience" issue. Not only that, it is to drive through a measure (human/animal embryo chimeras) not featured in New Labour's 2005 election manifesto.

We have become accustomed to the two faces of Gordon Brown, the authoritarian control-freak and Macavity, who is never there when things go wrong. The metamorphosis of one into the other represents a manifestation of the "the Caledonian antisyzygy" to match anything in James Hogg or Robert Louis Stevenson: Dr Stalin and Mr Bean - or Confessions of a Justified Whipper?

Since there is a crisis with bad publicity, we can expect the emergence of Macavity: Gordon will not be seen or heard. He will delegate a climb-down on this issue to the chief whip or other subordinate. On the other hand, there is a powerful challenge to his personal decision on a three-line whip. So, Gordon Stalin will ruthlessly assert his authority with an iron fist, leaving blood on the cabinet floor. Which will it be? But, hey, this is New Labour. There must be a third way.

Thomas McLaughlin, Jordanhill, Glasgow.

Once again a religious leader rants about the creating of human/animal embryos. He claims that man is unique and distinct from all other animals, although the evidence does not support his theory. Our closest relative in the animal kingdom, the chimpanzee, shares 98% of his genome with us. We can quantify the number of mutations that have occurred since it was 100% - ie, since we were both one and the same species. We know the average rate at which mutations occur and the average length of one generation, so it is possible to calculate that our two species diverged about six million years ago somewhere on the plains of Africa where the archaeological evidence is found.

Here an ape-like creature gave birth to two daughters; one produced a lineage which over the centuries underwent many mutations and formed several discreet species. After about a quarter of a million generations, only one survives, Homo sapiens, us. Meanwhile, her sister also gave rise to a lineage that eventually produced Pan troglodyte, the chimp. These facts may be unpalatable to some but they are incontrovertible.

It has been known for some time that new tissue and organs can be grown from stem cells obtained from embryos. Animals can provide a plentiful supply of fertilised eggs and when the nucleus is removed from one of these and replaced with the nucleus from the cell of a patient, the resulting embryo will provide stem cells from which can be grown healthy tissue and organs, compatible with the patient's own tissue and thus accepted by the immune system.

These cells are almost completely human since only the supporting cytoplasm and organelles are of animal origin; all the nuclear DNA is human. The research that could provide cures for many distressing and often fatal diseases has been delayed for years by influential religious leaders holding opinions that have no foundation in fact. This situation must not be allowed to continue; the many thousands of sufferers have waited long enough.

Clare Marsh, 272 Bath Street, Glasgow.

While conceding the right of Cardinal Keith O'Brien, leader of Scotland's Catholics, to spout forth his revulsion at this forthcoming Bill (The Herald, March 22), I despair at the description given as monstrous and likening it to Frankenstein.

I disagree with such utterances as they are totally scaremongering in content and without real substance. His homily to his faithful on Easter Sunday should have been about a Risen Christ, not about embryonic research; a subject he probably knows little about in scientific terms. I know if I had been in attendance at St Mary's Cathedral yesterday, it would have been the Easter message of Triumph over Evil that I should have hoped to have heard, the very essence of the Christian Gospel. I was fortunate enough to be in my place in my small but vibrant Congregational Church, listening to such a message.

Recently, we also had the unedifying spectacle of the Bishop of Motherwell misreading the pulse of 21st-century Scotland with a speech that should never have been written, let alone released for public consumption: full of intolerance and lacking in compassion against a certain section of humanity.

I know many Catholic friends, and they find their church, at times, in a dinosaural mode when it comes to looking forward in a changing world, especially with experimentation in the field of scientific knowledge which might give cause to the eradication of all sorts of diseases that might or might not some day have an impact on any family.

None of us is immune; not even cardinals or bishops. I applaud Jim Devine, Labour MP for Livingstone, a Catholic, who feels so strongly about the positive aspects of this Bill that he, unlike some elected members on all sides of the Westminster fraternity, is prepared to put his head above the parapet and declare his intentions on this Bill.

MPs should remember when those eligible to vote make their way to the polling booth, they don't vote on religious grounds but on a manifesto offered at the time. Cardinal O'Brien would do well to remember that intemperate language allied to a sort of arrogance cuts no ice with the a vast number in today's Scotland. He certainly doesn't speak for me.

Nelson Corbett, Cumnock.

In this argument which has been reduced to science v religion, it is important to point out that Cardinal O'Brien has a degree in chemistry from Edinburgh University, so his remarks are better informed than some people think.

Veronica Gordon Smith, Edinburgh.

WHY should the Prime Minister should allow a free vote on the Human Fertilisation and Embryology Bill? If MPs are allowed to vote on matters of policy on the basis of religious affiliation, then the electorate will feel compelled to take into account the religious affiliations when choosing an MP. It is difficult to imagine a worse outcome in a country still struggling to rid itself of sectarianism.

Paul Braterman, Professor Emeritus, University of North Texas; Honorary Senior Research Fellow in Chemistry, University of Glasgow.