RAG1 and acute lymphoblastic leukemia: This single base substitutions (SBS) signature was highly reproducible across all RAG-1 KO leukemia samples, including 7 BCP-ALL and 1 pre–T-LBL (8660_Thymus) (supplemental Figure 4A), with a consistent preference for the following order: CCG > GCG > ACG > TCG (Figure 4C; supplemental Figure 4A).