Genetic map of the non-recombining region of the human Y chromosome. The human Y chromosome consists of a large non-recombining region (NRY, which consists roughly of 24 megabases (Mb) euchromatin and 30 Mb heterochromatin) flanked by short pseudoautosomal regions (PAR, about 2.6 Mb and 0.4 Mb, respectively). Genes specific to the Y chromosome are indicated on the left-hand side of the diagram, and genes with homologs on the X chromosome are on the right-hand side. Ubiquitously expressed genes are shown in yellow, testis-specific genes are in blue, tooth-bud-specific genes in green, and brain-specific genes in red. Some testis-specific gene families have additional locations on the NRY not shown (m: multiple copies). The extensive region of Y-chromosomal heterochromatin is indicated by the grey area. The NRY is divided into intervals (as indicated by numbers on the chromosome), defined by naturally occurring deletions.