FANK1 (fibronectin type III and ankyrin repeat domains 1), also known as HSD13, is a 345 amino acid nuclear and cytoplasmic testis-specific protein found primarily in pachytene spermatocytes and round spermatids. Containing six ANK repeats and a single fibronectin type-III domain, FANK1 undergoes alternative splicing events to form three isoforms. Possessing DNA binding activity, FANK1 is suggested to act as a transcription factor and may regulate gene expression during spermatogenesis. The gene encoding FANK1 maps to human chromosome 10, which contains over 800 genes and 135 million nucleotides, making up nearly 4.5% of the human genome. PTEN is an important tumor suppressor gene located on chromosome 10 and, when defective, causes a genetic predisposition to cancer development known as Cowden syndrome.