JAK2 (janus kinase 2) is a non-receptor protein tyrosine kinase involved in the interferon-alpha/beta/gamma pathway and is a member of the JAK/STAT signaling pathway. Simple variants in JAK2 are seen in about 31% of all tumors, most of which are hematopoietic. JAK2 variants are found in about 1% of breast tumors, colon tumors, and lung tumors, and 2% of skin tumors. Preclinical studies have shown response with JAK2 inhibitors in breast cancer models.