The human Y chromosome contains 60 million base pairs (Mb) of DNA, it is haploid, and 95% of it is nonrecombining. Helen Skaletsky from the Whitehead Institute for Biomedical Research and colleagues report in the first of two papers in the June 19 Nature that the 23Mb euchromatic region in the Y chromosome comprises eight massive palindromic sequences and that these regions are rich in genes that are functional and testis-specific (Nature, 423:825-837, June 19, 2003).

In the second paper, they describe both comparative sequencing of the great ape Y chromosome, and the mechanism of gene conversion by which the Y chromosome repairs mutations that occur within these genes (Nature, 423:873-876, June 19, 2003). The results raise important questions about the molecular clock dating of segmental duplications in the human genome and the rate of human–chimpanzee divergence in these regions.

Skaletsky et al. sequenced 97% of the...