Reduced number of HSCs are present in the bone marrow of Anxa2-deficient (Anxa2^−/−) mice. Competitive long-term bone marrow transplantations (CLT-BMT) were used to determine HSC frequency in murine wild-type or Anxa2^−/− mice. Lethally irradiated Ly-5.1 mice (570 cGy twice in 3 hours) were injected intravenously with 2 doses bone marrow mononuclear cells derived from the Anxa2 knock-out or wild-type animals together with a radioprotective dose of 2 × 10⁵ bone marrow cells (Ly-5.1). The proportions of Ly-5.2 cells in peripheral blood of the different lineages (MAC-1, B220, Gr-1, or CD3 cells) were analyzed by 4-color flow cytometry at 16 weeks. The variation in the proportion of animals not displaying Ly5.2-positive cells (0.1% cut off) in the peripheral blood was analyzed by Poisson statistics to measure HSC frequency. The Anxa2^−/− animals had nearly a log-fold fewer HSCs per 10⁶ mononuclear marrow cells compared with age-matched wild-type animals.