Abbreviated “HSC” and often somewhat inaccurately used interchangeably with hematopoietic progenitor cell (HPC), this term describes very early cells present normally in small quantities in the bone marrow and even smaller quantities in the peripheral blood. HSCs are totipotent, meaning that they can develop into and populate any of the three cell lines (erythroid, myeloid, or megaloblastic). HPCs, on the other hand, are committed to a single cell line.