Broad alterations in the rate of gene transcription are required for malignant transformation. As a consequence, cancer cells are vulnerable to perturbations of transcriptional regulators, including DNA-binding transcription factors and chromatin proteins. Our laboratory has taken a functional-genomics approach to identify essential transcriptional regulators in cancer with “oncogene-like” properties. Our initial efforts used RNAi screening to identify the BET bromodomain protein BRD4 as an actionable dependency in acute myeloid leukemia. Over the past few years, we have advanced our mechanistic understanding of BRD4 as a therapeutic target in leukemia by revealing the upstream and downstream factors that support BRD4-dependent transcriptional activation. More recently, we have pursued the use of domain-focused CRISPR-Cas9 screening as a tool for annotating essential transcriptional regulators and signaling molecules in cancer, which exploits the heterogeneity of Cas9-induced indel mutations to infer structure-function relationships of individual protein domains.