JARID1B (Jumonji AT-rich interactive domain 1B) is a member of the JARID 1 family of proteins that catalyze the demethylation of histones on lysine. The Jarid 1 family members include JARID1A, JARID1B, JARID1C and JARID1D. The members of this family possess a Jumonji C (JmjC) and Jumonji N (JmjN) domain as well as one ARID (AT-rich interactive domain) and three PHD-type zinc fingers. As histone demethylases, JARID 1 proteins function as transcriptional repressors. JARID1B expression is largely restricted to testis and may play a critical role in tumorigenesis as it has been found to be upregulated in prostate and breast cancer.