Ex vivo expanded CD34(+) hematopoietic stem and progenitor cells (HSPCs) have compromised homing and engraftment capacities. To investigate underlying mechanisms for functional changes of expanded HSPCs, we compared gene expression profiling of cultured and fresh CD34(+) cells derived from cord blood using SMART-PCR and cDNA array: 20 genes were up-regulated while 25 genes were down-regulated in cultured CD34(+) HSPCs. These differentially expressed genes are involved primarily in proliferation, differentiation, apoptosis, and homing.