When the BBC reported earlier this year that American scientists had injected human stem cells into pigs in the hope of growing organs for transplant, some Jews inevitably baulked at the word "pig".
But there is no difficulty in Jewish law. "It is forbidden to eat pig and to cultivate it [for food]," says Rabbi Professor Avraham Steinberg. "But there is no prohibition to derive benefit… for the sake of curing an illness."
Heart valves in pigs are already used in surgery. If the new stem cell technology works one day, then even the strictest Orthodox rabbi would happily take a pig-grown organ if his life depended on it. "One can envision hospitals will have a pig stall and they will sacrifice the pigs every day to save lives," he says.
For years, Rabbi Steinberg has been one of the go-to men in Jewish medical ethics in a field where scientific advances are constantly posing new questions for halachah. Born in 1947, he still works as a paediatric neurologist at Shaare Zedek hospital in Jerusalem and combines his medical expertise with encyclopedic Jewish knowledge.
He won the Israel Prize for his authorship of the Encyclopedia of Jewish Medical Ethics. Now he is chief executive of the Talmudic Encyclopedia, an extraordinary enterprise begun in 1942 when rabbis in pre-state Israel feared for the survival of Jewish resources in Europe. With 37 volumes published, he hopes his team will complete the remaining 30-odd volumes in 10 years.
In some cases, rabbis may allow parents to choose the gender of their child
In great demand as a speaker across the Jewish world, he was over here earlier this month as scholar-in-residence at the Village Shul in Hampstead.,
Stem cells provide one of the great hopes of contemporary science. They have the capacity to develop into any of the 200 various types of tissue in the body: heart, liver, eye etc. If researchers can discover the process that leads one stem cell to develop into a heart, another into an eye, then the door is open to growing organs for transplant instead of relying on donors, who are in short supply. Stem cells may also be able to replace dying cells in patients with degenerative diseases like Alzheimer's or Parkinson's.
The religious question arises over their procurement. "The best way is to get it from a fertilised egg that has been developed for four or five days," Rabbi Steinberg says.
When couples having fertility problems go to an IVF clinic, the procedure is for the women to produce a number of fertilised eggs; one or two will be implanted back into the woman, leaving several spares. "If we extract the stem cells from a fertilised egg, we kill the egg," Rabbi Steinberg explains. "Is it a form of abortion or killing?"
For the Catholic Church, this is a barrier, as it regards human status to begin with the moment of conception. "Judaism has a very different outlook," says Rabbi Steinberg. "From a Jewish point of view, a not-yet-implanted fertilised egg whose age is less than 40 days has no human status. Hence if you destroy it, you are not killing or aborting it."
In order to save lives, stem cell research is thus "absolutely permissible".
One technique that is now available is pre-implantation genetic diagnosis. A couple may have given birth to a child with a severe genetic disease such as Tay Sachs or cystic fibrosis and want to avoid it in a second child. Since the genes that cause some diseases can be identified, if the flawed gene is removed, then the baby can be born healthy.
The couple can have an IVF procedure and a fertilised egg produced. When there are just a few cells in the egg, one cell can be removed. If the faulty gene is found in the egg, then the egg can be discarded. But if it is free of the rogue gene, then the egg can be implanted in the mother and the family rest assured the baby will be born without that condition.
"Are we permitted to do this testing and discard this fertilised egg that does carry the sick gene? Halachically, since it is outside the womb and less than 40 days old, you are allowed to discard it," says Rabbi Steinberg.
PGD can be used in Jewish law only for medical treatment. Even though the gene determining eye colour can now be identified, rabbis would not sanction its use for cosmetic or social purposes – for example, if it were possible to genetically enhance intelligence.
But rabbis may allow usage of PGD to determine the child's gender in cases where there is risk of a serious genetic disease that affects only males.
Genetic research may have spin-offs beyond medicine. One project could even help decide Jewish status. A leading Israeli rabbi is currently looking at a study which found that a high percentage of Ashkenazi women carry particular mutations in their mitochondria (which produce energy for cells in the egg). By contrast, only two out of thousands of non-Jewish women in the study showed the same genetic feature.
Since mitochondria is transmitted through the maternal line - as is Jewish status- this could be used to determine who is a Jew or not. For women without documentary proof of Jewishness, as in the case of many from the former USSR who have emigrated to Israel, such biological proof could one day be accepted by Orthodox rabbis.
As for the two non-Jewish women in the sample with the mitochondrial twist, Rabbi Steinberg suggests they may be Jewish in origin - but simply don't know it.