Keratin 40 (KRT40), also known as Cytokeratin-40 (CK-40) or KA36, is a 431 amino acid protein that belongs to the intermediate filament family. As a heterotetramer of two type I and two type II keratins, Keratin 40 may play a role in late hair differentiation. While weakly expressed in tongue, breast, colon and small intestine, Keratin 40 is highly expressed in skin and scalp. The gene that encodes Keratin 40 consists of approximately 9,420 bases and maps to human chromosome 17q21.2. Encoding more than 1,200 genes, chromosome 17 comprises over 2.5% of the human genome. Two key tumor suppressor genes are associated with chromosome 17, namely, p53 and BRCA1. Malfunction or loss of p53 expression is associated with malignant cell growth and Li-Fraumeni syndrome. Like p53, BRCA1 is directly involved in DNA repair, though specifically it is recognized as a genetic determinant of early onset breast cancer and predisposition to cancers of ovary, colon, prostate gland and fallopian tubes.