CASP3 and anemia (phenotype): Subsequent studies found CD34+lin-Sca-1+c-kit+ cells, which are mostly composed of multipotent progenitor cells (MPPs) and hematopoietic progenitor cell 1 and 2 (HPC1 and 2)7,8, as the immature progenitor that migrates from the bone marrow to the spleen in response to anemia and produce BMP-responsive BFU-E4,9.