PECAM1 and autoimmune lymphoproliferative syndrome: We did not find any difference in the proportion of recent thymic emigrants as measured by expression of CD31 on naïve CD4 T-cells nor any significant change in the proportions of naïve or memory CD4 T cells, or in the proportion of cycling CD4 T cells in the CD4 lymphopenic patients with ALPS-FAS compared to matched controls with ALPS-FAS and normal CD4 cell counts.