We functionally validated these findings with isogenic CRISPR/Cas9 generated models of del(11q) together with TP53 and/or ATM mutations, showing that CLL cell lines harboring biallelic loss of ATM and TP53 present abnormal cell cycle and mitotic profiles, failing to engraft and compete in murine xenotransplants. Here, ATM is linked to B-cell chronic lymphocytic leukemia.