T‐ALL leukemia, which originated from either pro‐B or GC cells, showed clonal TCR rearrangements (Fig 5E) and a significant similarity to Rosa26‐Lmo2 + Sca1‐Cre tumors at the genomic level due to the presence of recurrent Notch1 (SNVs (3/4), indels (1/4) in Rosa26‐Lmo2 + Mb1‐Cre; SNVs (1/1), indels (1/1) in Rosa‐Lmo2 + Aid‐Cre) mutations (Table 1; Table EV1). Here, CD79A is linked to acute lymphoblastic leukemia.