FACS analysis of CD11b+cells and Gr1+CD11b+MDSCs. Ly-6G (Gr1)+, CD11b+, and Gr1+CD11b+ co-positive cells were analyzed in single-cell suspensions prepared from untreated (UT) and metronomic CPA-treated (CPA) spleens, bone marrow and 9L tumors from scid mice euthanized 6 days after the 4th CPA cycle (day 24). Cell numbers in each quadrant are expressed as a percentage of the total cell population. Metronomic CPA significantly increased single CD11b-positive populations in spleen and bone marrow (p < 0.05) and tumor (p < 0.001), but decreased Gr1-CD11b co-positive populations in bone marrow (by 2-fold; p < 0.05) and spleen (by 4.7-fold; p < 0.001) (n = 2 per treatment group), with no significant increase in treated tumors (n = 4). IgG background for Gr1 (spleen: 0.06%, bone marrow: 0%, and tumor: 0.01%), CD11b (spleen: 0.22%, bone marrow: 0.11%, and tumor: 0.34%), and Gr1-CD11b co-positive (spleen: 0.06%, bone marrow: 0.02%, and tumor: 0.02%). Also see Additional file 1. Each treatment group was repeated at least 2–3 times.