It is estimated that about 10% of the genes encoded by the X chromosome are associated with a family of "CT" genes, so named because they encode for markers found in both tumor cells (in cancer patients) as well as in the human testis (in healthy patients).

What does the X chromosome do?

The X chromosome determines your sex, gives some females super color vision and lends its magic to a certain breed of cat.

What was X chromosome called then?

Like Henking, McClung used “x” to label this chromosome in the figures of his 1899 paper, but he coined the term “accessory chromosome” to identify it from then on. McClung soon hypothesized that the accessory chromosome determined male sex.