Q: Last year the final human genome sequence was announced. What do we now know about the human genome from this international sequencing effort?

A: People used to think that there were about 100,000 genes in the human genome. It turns out that there are only roughly 30,000 genes, but those genes may make millions of proteins because each gene can be read in different ways. Of the 3 billion base pairs in the human genome, only 5 percent looks like it is important. Of that 5 percent, only half codes for proteins. The remaining 2.5 percent regulates when and how those genes are turned on. Even identifying that 5 percent of the genome turns out to be nontrivial, and that's just building the parts list for the genome. Then we want to find out what those genes do. I think it's going to take us 100 years to read this book of life.

Q: What about the remaining 95 percent of the genome?

A: We don't know. It could be that it's just important to have a certain amount of space in the genome, or that DNA could play a role in replication, cell division or the structure of the DNA. Some of it may also play a role in regulating genes.

Q: How do you tell which parts of the genome are important?

A: The important parts are conserved in different species. If it's a sequence that makes a critical protein, it will remain similar even in distantly related animals. Less important regions of the genome have a lot of variability. That's one reason why it's important to sequence organisms that aren't closely related to humans. If you sequence only humans and chimpanzees, which are our closest relatives, the entire sequence will be quite similar and you can't learn which sequences are most important. I'm on a government committee along with other professors at Stanford including Arend Sidow and David Kingsley to decide which other organisms should be sequenced. You want to look at the evolutionary tree and make sure you have representative animals from all the branches, even if they seem kind of obscure. There's a lot of interesting biology to be learned from sequencing marsupials, for example.

Q: How has the human genome changed the way science is done?

A: We used to ask questions one gene at a time. Now you can think about experiments on a large scale. We're not just looking at regulatory sequences for one gene. We're looking at those sequences for all genes. We're not just asking what role one gene plays in development. We're asking how all genes interact. These types of experiments wouldn't be possible without sequence data.

Q: What are some interesting questions in the field of genetics?

A: The hot new problem has actually been a problem since scientists have been studying biology: Why does this cell do one thing and this cell do another? The liver cell and nerve cell have exactly the same DNA but why are they so different? A big part of answering that question is to figure out what genes are turned on in the different cells and how those genes are regulated. Even though this research has been going on for a long time, we're now approaching the question in a different way. We can now look at the expression and behaviors of all genes in each cell type to learn how they are different.