Updated project metadata. The ATR-CHK1 signalling pathway responds to single strand DNA breaks and is required for cancer cells to survive the high levels of DNA replication stress and genomic instability resulting from the activity of oncogenes such as MYC. For this reason, inhibitors of CHK1 have great potential as anti-cancer drugs and are currently in clinical trials. However, overcoming de novo or acquired resistance to CHK1 inhibition is required if these are to be used as effective therapies. We discovered that lymphomas from the E-Myc mouse model of MYC driven B-cell lymphoma, with a deletion of the c-Rel NF-B subunit, have defective CHK1 activity and are resistant to treatment with the highly specific CHK1i CCT244747. Here we use quantitative (phospho)proteomics approaches to investigate how de novo resistance to CHK1i is acquired and can be overcome, offering potential therapeutic opportunity in patients developing resistance to Chk1i in the clinic.