CUX1(CUTL1, CDP) is a transcription factor proposed to act as a haploinsufficient myeloid tumor suppressor that maps to the commonly deleted segment in cases of myeloid malignancies with complete or partial loss of chromosome 7; such cases show reduced expression of CUX1. In addition, loss of function (missense, nonsense, frameshift) mutations of CUX1, occuring throughout the gene, have been described in a variety of cancer types; they occur in up to 10% of chronic myelomonocytic leukemia and are rare (less than 5%) in acute myeloid leukemia, myelodysplastic syndromes, myeloproliferative neoplasms and chronic lymphocytic leukemia. When loss of function mutations do occur in CUX1, they do not coexist with monosomy 7 or del7q, consistent with its role as a haploinsufficient tumor suppressor gene in most cases; nevertheless, compound heterozygous loss of function mutations in CUX1 have been described in rare cases. In MDS and MDS/MPN, both CUX1 inactivation and −7/del(7q) have been associated with poorer overall survival according to some studies. CUX1 deficiency has been reported to activate phosphoinositide 3-kinase (PI3K) signaling through direct transcriptional downregulation of the PI3K inhibitor PIK3IP1 (phosphoinositide-3-kinase interacting protein), leading to increased tumor growth and susceptibility to PI3K-AKT inhibition in some models.