Updated publication reference for PubMed record(s): 28554894. Sin3a is the central scaffold protein of the prototypical Hdac1/2 chromatin repressor complex, crucially required during early embryonic development for the growth of pluripotent cells of the inner cell mass. Here, we explore the endogenous composition of the Sin3a-Hdac complex in pluripotent embryonic stem (ES) and differentiated cells. To do this, we established an endogenous double immunoprecipitation strategy coupled with quantitative mass spectrometry (ENDIP-MS) allowing us to define the precise composition of the Sin3a complex in multiple cell types. We identify the Fam60a subunit as a key defining feature of a variant Sin3a complex present in ES cells, but not in differentiated cells. Fam60a co-occupies H3K4me3 positive promoters with Sin3a and is essential to maintain it on chromatin. Consistent with this, Fam60a depletion phenocopies the loss of Sin3a, leading to decreased proliferation, an extended G1-phase and the deregulation of genes associated with differentiation. Taken together, our data characterise Fam60a as an essential core subunit of a variant Sin3a complex in ES cells required to promote rapid proliferation and to prevent unscheduled differentiation.