SRP134014 PRJNA437106 GSE111457 SLAM-seq defines direct gene-regulatory functions of the BRD4-MYC axis [Quant-Seq] Defining direct targets of transcription factors and regulatory pathways is key to understanding their role in physiology and disease. Here we combine SLAM-seq, a novel method for direct quantification of newly synthesized mRNAs, with pharmacological and rapid chemical-genetic perturbation to interrogate primary transcriptional targets of BRD4 and MYC and define the response to BET bromodomain inhibitors (BETi). While BRD4 acts as a global co-activator of Pol2-dependent transcription in a BET bromodomain-dependent manner, therapeutic BETi doses deregulate a small set of hypersensitive target genes. In contrast to BRD4, MYC primarily acts as a selective transcriptional activator that controls basic metabolic processes such as ribosome biogenesis and de-novo purine synthesis across diverse cancer contexts. Beyond defining primary regulatory functions of BRD4 and MYC in cancer, our study establishes a simple, robust and scalable approach to dissect direct transcriptional targets of any gene or pathway. Overall design: Baseline gene expression profiling of clonal cell lines engineered to express AID-tagged proteins and unedited controls by 3''mRNA sequencing. GSE111457 parent_bioproject PRJNA437109