We have found that the USP37 deubiquitylase binds to the DNA helicase known as CMG (Cdc45-Mcm-GINS) in mammalian cells. CMG unwinds the parental DNA duplex during the process of chromosome duplication, and USP37 is important to help cells survive problems in DNA replication. In this project, we investigated the partners of the mammalian CMG helicase. We compared control mouse embryonic stem cells (mES cells) to mES cells in which the SLD5 component of the CMG helicase was fused to the ‘Tandem Affinity Purification’ tag (TAP-tag). Cells were either untreated or exposed to 5 µM CB-5083 for three hours, to inhibit activity of the p97 ATPase.