In previous studies, deletion of RIP1 in germ cells resulted in perinatal lethality,10 and few mature T cells were generated from RIP1−/− fetal liver cells.15, 26 More recently, studies found deletion of RIP1 in HSCs/Ps led to lymphocyte deficiency.27 However, it remains unclear what role RIP1 has in T cells during postlineage commitment stages. Here, RIPK1 is linked to hyperinsulinemic hypoglycemia, familial, 4.