CT45A4 is a 189 amino acid protein expressed in testis and in various cancer cell lines. The CT45 family comprises six highly similar (>98% cDNA identity) genes that are clustered in tandem within a 125-kb region on Xq26.3. It is suggested that CT45 is significantly phosphorylated and down-regulated during mitosis. Localized to the nucleus, CT45 may be involved in the malignancy of classical Hodgkin lymphoma in patients and is considered to be a potentially valuable cancer target. CT45 is encoded by a gene located on human chromosome X, which consists of about 153 million base pairs and nearly 1,000 genes. 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.