Distinct development of SP4 (CD4+CD8-CD25+/-) cells and FoxP3 gene expression in new born thymus organs. A: Comparable levels of CD4+CD8-CD25+ thymocytes are present in day 1–8 neonatal and adult thymus. Neonatal thymus organs were analyzed for CD4, CD8 and CD25 by FACS. Relative percentages of CD4+ thymocytes and CD4+CD8-CD25+ thymocytes are indicated. Shown are neonatal mice from a representative litter of at least three independent experiments. B: FoxP3 mRNA is not expressed in neonatal thymocytes until 3 days after birth. Total cells from the newborn thymus were harvested at indicated times after birth and analyzed for relative expression of FoxP3 (FoxP3/18S) by real-time PCR. SD is derived from duplicate samples and shown as error bars. At least 4 litters of newborn mice were analyzed with similar results.