The jumonji domain containing 1B protein (JMJD1B) was originally discovered as a candidate for the myeloid leukemia tumor suppressor gene. Besides CD34+ cells and acute myeloid leukemia (AML) cell lines, JMJD1B mRNA is highly expressed in liver, heart, kidney, skeletal muscle, and placenta tissues. The JMJD1B gene is localized to a region of chromosome 5q31, which is frequently deleted in myeloid leukemias and myelodysplasias and expression of JMJD1B in a del(5q) cell line results in suppression of clonogenic growth suggesting that JMJD1B may function as a tumor suppressor. In contrast, JMJD1B gene copy number and mRNA expression level was increased in several non-small cell lung cancers indicating that the role of JMJD1B in cancer formation and progression is more complex than originally postulated. At least three isoforms of JMJD1B are known to exist.