S1P-R1 protein is expressed to the greatest extent on the most mature thymocyte subset. Human postnatal thymocytes were stained with surface antibodies (see Table E1). A, S1P-R1 profile of 1 representative postnatal (2-year-old) thymus gated as described above. B, Summary of S1P-R1 expression on cells from 14 postnatal thymi (median age, 9 months) showing comparison of S1P-R1 expression on CD27+CD3hiCD69− thymocytes to less mature populations. C, Expression of S1P-R1 on fetal thymocyte populations (median age, 16.5 weeks of gestation). For statistical data, please see Table E3.