HOXA9−/− animals have reduced total white blood cell counts, granulocytes, and B and T lymphocytes. Peripheral blood cell counts were determined for (□) HOXA9 homozygotes (n = 22), () heterozygotes (n = 12), and normal parental strains including (▪) C57B1 (n = 24), (▧) 129 Sv/Ev (n = 18), and (▧) F1 hybrids of C57B1 and 129 Sv/Ev (n = 20). Animals were 9 to 12 weeks of age. WBC, total white blood cell counts; Grans, granulocytes; lymphs, lymphocytes. All counts expressed as cells/mm3 (mean ± SE). B- and T-cell counts were determined by FACS analysis using antibodies to B220 and CD3 on samples from 8 to 9 animals per group. Leukocyte counts of HOXA9−/− animals were compared with those of each group of control animals using a 2-tailed Student's t test. Control counts were significantly different from HOXA9−/− counts (P < .05), except those marked with a plus sign (+).