KMT2A and leukemia: To confirm that leukemia cells expressing the R54 epitope at high levels are resistant to OVA-specific CTLs, we FACS-sorted OVA-expressing MLL/AF9 leukemia cells into R54high and R54low subpopulations expressing comparable levels of OVA-ires-GFP and H-2kb, and then co-cultured them with OVA-specific T cells (Fig 5A).