Expression levels of Hoxa9 and Pim1 track together in early hematopoietic compartments. (A) FACS sorting strategy. Normal c-kit–enriched murine marrow cells (i) were FACS sorted into myeloid progenitor (MP; Lin⁻ c-kit⁺ Sca-1⁻) and KLS (Lin⁻ c-kit⁺Sca-1⁺) fractions (ii). The KLS fraction was further separated into LT-HSC (flk-2⁻) and ST-HSC/MPP (flk-2⁺) fractions (iii), and the MP fraction was further enriched for the CD34⁺ CMP/GMP populations (iv). (B) mRNA levels of Hoxa9 and Pim1 in primitive hematopoietic compartments. RNA isolated from the 3 fractions described in panel 4Ai-iv was subjected to quantitative real-time PCR (Q RT-PCR) as described in “Assays for myeloid progenitors and proliferation assays,” under “Materials and methods,” and expression levels were normalized to GAPDH mRNA levels. Both Hoxa9 and Pim1 show their highest level of expression in the LT-HSC fraction, with progressively declining levels in more differentiated fractions (*P < .01 comparing LT-HSC or ST-HSC/MPP fractions vs CMP/GMP fractions).