CDKN2A and leukemia: Because mouse BCR-ABL+ Arf−/− leukemia cells, like human primary B-ALL cells17, lack the vitamin D receptor (VDR) (Supplementary Fig. S8) required for mediating the VD3 signal, and because leukemic cells proliferate in the bone marrow, BCR-ABL ALL cells were co-cultured with immortalized human mesenchymal stromal cells (hMSCs) expressing VDR that mimic a bone marrow micro-environment.