CD34 and leukemia: In order to investigate the role of HCK on CXCL12/CXCR4 axis in leukemia cells, two human myeloid leukemia cell lineages, the U937 (AML CD34 negative cells) and the KG1a (AML CD34 positive cells) were stably transduced with lentivirus-mediated shRNA targeting HCK gene (shHCK) or an appropriate control (shControl).