The probability of finding insertional mutations in both genes by chance in the same leukemia is exceedingly small and has led to the suggestion that Il2rg is a leukemia gene that cooperates with Lmo2. While IL2RG is not overexpressed in SCID-X1 leukemias [3] or in the mouse leukemia with an Il2rg insertion, subtle effects on its expression, such as an inability to downregulate its expression during T cell development, could be oncogenic [10]. The gene discussed is IL2RG; the disease is leukemia.