It could be vasal vagal syncopy.
Nobody knows how many there are.
So far the wagers by the people who are likely to know range wildly from 27,000 all the way to 200,000. The wagering will remain open until 2003 because one thing genome scientists do know is that the human DNA sequence isn't anywhere near finished yet. Even when it is, picking out genes from the mass of other DNA will be very tough. On the other hand, the exact number may not matter much. This we have now learned from investigating the genes of other creatures.