Gene regulatory networks are pivotal for many biological processes. In mouse embryonic stem cells (mESCs) the transcriptional network can be divided into three functionally distinct modules: Polycomb, Core and Myc. The Polycomb module represses developmental genes, while the Myc module is associated with proliferative functions and its mis-regulation is linked to cancer development. Here, we show that in mESCs the Polycomb Repressive Complex 2 (PRC2) associated protein EPOP (a.k.a C17orf96, esPRC2p48, E130012A19Rik) co-localizes at chromatin with members of the Myc and Polycomb module. EPOP interacts with the transcription elongation factor Elongin BC and the H2B deubiquitinase USP7 to modulate transcriptional processes in mESCs similar to MYC. EPOP is commonly up-regulated in human cancer and we demonstrate that its loss impairs the proliferation of several human cancer cell lines. Our findings establish EPOP as a unique modulator of transcriptional processes, impacting both Polycomb and active gene transcription in mammalian cells. Overall design: Genome-wide investigation of the role of EPOP in mESCs and SH-SY5Y cells