Sex determination is accomplished in a great variety of ways, including chromosomal (XY and ZW systems), genetic (GSD) and environmental sex determination (ESD). How these systems relate to each other has been a matter of scientific debate for more than a century. A conserved sex determining gene and conserved sex chromosomes seem unlikely in vertebrates, yet the sex chromosomes of birds and snakes (which are not closely related) appear superficially almost identical. Sex determination is a complex process and the most important developmental decision for an individual. The study of sex chromosomes and the genes they hold is the basis for understanding this fundamental process. We are using a variety of reptile species to study the genetic relationships of their sex chromosomes and the genes involved in sex determination. Broadly our aims are to i) discover the molecular and cytological relationships of sex chromosomes among reptiles ii) discover the ancestral chromosome pair(s) that became sex chromosomes in birds and snakes iii) discover novel sex determining genes in reptile and bird sex determining pathways. We are using repetitive DNA elements to investigate the relationships of the heteromorphic sex chromosome in birds and snakes. We have discovered a novel class of repeats common to the chicken and snake W, though their accumulation on the W in the derived lineages of snakes is greatly advanced. We are investigating the cytological distribution of the banded krait minor (Bkm) satellite repeat among snakes. Its differential accumulation on the Z and W chromosomes makes it a candidate for a PCR based molecular sexing test. We have developed the first such molecular test for sex in snakes.