ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: It has been postulated that expression and signaling of CXCR4 are modulated by the level of BCR-ABL [51] and higher expression of BCR-ABL both in mRNA and protein levels in CML stem cells [52] may cause downregulation of CXCR4 and release of CML stem cells into the peripheral blood.