These shared characteristics between the P440S mice and the patients include (1) a 20- and 50-fold reduction in Lck protein expression by splenic CD4+ and CD8+ T cells, respectively (Fig. S3 A); (2) severe (splenic) T cell lymphopenia (Fig. 3, A and B); (3) decreased CD4 and CD8 expression by T cells (Fig. 3, C and D); and (4) skewed CD4+ T cell memory phenotype (Fig. 3, E and F). Here, LCK is linked to lymphopenia.