HDX (highly divergent homeobox) is a 690 amino acid nuclear protein that contains two homeobox DNA-binding domains. Existing as two alternatively spliced isoforms, the gene encoding HDX maps to the human X chromosome, which consists of about 153 million base pairs and nearly 1,000 genes. The combination of an X and Y chromosome leads to normal male development while two copies of X lead to normal female development. There are a number of conditions related to an unusual number and combination of sex chromosomes being inherited, including Turner's syndrome, Klinefelter's syndrome and Triple X syndrome. Color blindness, hemophilia, and Duchenne muscular dystrophy are well known X chromosome-linked conditions which affect males more frequently as males carry a single X chromosome.