PKX1 appeared to encode a novel type of protein kinase that is related to the catalytic subunit of cAMP-dependent protein kinases and has striking homology to the DC2 protein kinase of Drosophila melanogaster. PKX1 is part of a family of at least 4 genes or pseudogenes, of which 3 map to the human sex chromosomes. In contrast to all other genes from the X-specific region of Xp22.3, PKX1 has a homolog on Yp (PRKY) rather than Yq. This suggests that the single pericentric inversion event hypothesized to have occurred during primate evolution is not sufficient to explain the present X/Y homology pattern of Xp22.3.This suggests that the PKX1 gene, besides harboring a previously described hotspot for illegitimate Xp/Yp recombination, contains additional sequences which predispose the X chromosome to breakage events.