The molecular chaperone Hsp70 folds a significant proportion of the proteome and is responsible for the activity and stability of many disease-related proteins, including those in cancer. Substantial effort has been devoted to developing a range of chaperone inhibitors for clinical use. Recent studies have identified the oncogenic ribonucleotide reductase (RNR) complex as an substrate of chaperones. While several generations of RNR inhibitor have been developed for use in cancer patients, many of these produce severe side effects such as nausea, vomiting and hair loss. Development of more potent, less patient-toxic anti- RNR strategies is highly desirable. Here we identify the yeast Hsp70 co-chaperone Ydj1 as an interactor of the RNR complex. Ablation of Ydj1 function destabilizes Rnr2 and Rnr4. We demonstrate conservation of this mechanism in mammalian cells, where HDJ2 interacts with and stabilizes R2 and R2B. Inhibition of HDJ2 with a novel small molecule inhibitor (116-9e) sensitizes cancer cells to existing RNR therapies. Going forward, this may form part of a novel strategy to target cancer cells that are resistant to standard RNR inhibitors. Following on from these discoveries, we examined the respective roles of yeast Hsp70 isoforms (Ssa1, 2, 3 and 4) in activating RNR. Interestingly, while Ssa1 and 2 can support RNR function, Ssa3 and 4 cannot. Biochemical analysis reveals that Ssa3 and 4 have a substantially weaker binding to RNR subunits than Ssa1 and 2 and indeed Ydj1 itself displays a lower binding for RNR in cells expressing on Ssa3 and Ssa4. Taken together, this adds to the growing body of evidence that Hsp70 isoforms have both overlapping and unique functions in cells. BiP, the Hsp70 isoform located in the Endoplasmic Reticulum has been shown to be important in cellular homeostasis and also important in cancer cell progression. Although several BiP inhibitors have been developed, they have not succeeded in clinical trials due to toxicity. We examine the individual roles of BiP co-chaperones ERdj1-8 in mediating anticancer drug resistance using a combination of ERdj1-8 CRISPR KO cells and chemogenomic screening. Interestingly, we find that each ERdj KO displays a unique signature of synergy with currently used anti-cancer drugs. Overall these data may suggest a personalized medicine approach whereby ERdj mutation status is assessed to design an effective anticancer treatment plan.