PCNA and severe combined immunodeficiency: Consistently, modulation of the levels of EPHB6 in colon cancer cells did not affect their growth when they were injected subcutaneously in NOD/SCID (nonobese diabetic/severe combined immunodeficiency) immunodeficient mice (Fig. 3I–L and Supplementary Figure 3) and no differences were observed in the proportion of proliferating cells (BrdU- or PCNA-positive) in the subcutaneous xenografts formed by cells with EPHB6 modulation compared to control cells (Supplementary Figure 4).