Conditional deletion of STAT5 increases host HSC apoptosis and decreases HSC pool size. BM from STAT5 KO and littermates was assayed by multiparameter flow cytometry to quantitate the number of primitive BM fractions. BM was stained with antibodies against lineage markers, c-Kit, Sca-1, and Flk2 (A) or lineage markers, c-Kit, Sca-1, CD34, CD16/32, and IL-7R (B,C). Three separate experiments were performed with 3 to 5 mice per genotype compared. (A) The absolute number of primitive HSC populations in BM cells from both hind limbs. KLS cells were defined as c-Kit⁺Lin⁻Sca-1⁺ cells. ST-HSCs are identified as Flk2⁺KLS cells and LT-HSCs as identified as Flk2⁻KLS cells. (B) The absolute number of common myeloid progenitor (CD34^+/lowCD16/32^intLin⁻c-Kit⁺Sca-1⁻), granulocyte-macrophage progenitor (CD34⁺CD16/32⁺Lin⁻c-Kit⁺Sca-1⁻), and megakaryocyte-erythroid progenitor (CD34⁻CD16/32⁻Lin⁻c-Kit⁺Sca-1⁻) cells per both hind limbs are shown. (C) The absolute number of CLP was defined by IL-7R⁺Lin⁻Sca-1^lowc-Kit^low phenotype and is shown per both hind limbs. (D) The proportion of annexin V–positive (DAPI-negative) cells within the ST-HSC and LT-HSC fractions is shown (n = 3). (E) Three to 4 months after pI:pC treatment (7 doses), the absolute number of LT-HSC defined both as CD34⁻ and Flk2⁻ KLS were analyzed from both hind limbs (n = 7). (F) One or 5 months after pI:pC treatment (7 doses), the percentage of annexin V–positive/DAPI-negative LT-HSCs (CD34⁻ KLS) was analyzed for wild-type (n = 6) and KO (n = 7) mice.