Although a few regulators of HOXA9 in MLLr leukemia have been previously identified (Zeisig et al., 2004; Collins and Hess, 2016b; Collins et al., 2014; Li et al., 2013a; Sun et al., 2013; Li et al., 2013b; Ogawara et al., 2015; de Bock et al., 2018; Lynch et al., 2019), to date a comprehensive CRISPR/Cas9 screen to unbiasedly identify novel upstream regulatory factors of HOXA9 has not been feasible owing to the lack of a reliable reporter cell line. Here, HOXA9 is linked to leukemia.