Figure 3. Possible hematopoietic stem/progenitor fates in the absence of STAT5. We have observed high levels of HSC/HPC engraftment in mice lacking STAT5. However, due to the strong multilineage selective growth advantage of the wild-type donor cells vs. the STAT5 knockout cells, it has been difficult to assess the impact in HSC vs. HPC compartments of the bone marrow. It is possible that only a subset of niches are vacated in the absence of STAT5 and that competition for these niches is not selectively impaired (homing, engraftment). STAT5 deficient HSC may not leave the niches as part of normal trafficking but rather prevent donor HSC from occupying niches. STAT5 deficient HPC may be vacated from putative HPC niches resulting in greater multilineage engraftment from multipotent progenitors.