Instead, this would target both healthy and cancerous cells. However, the idea is that the cancer cells are more susceptible to the drug. Healthy cells will be affected, but cancer cells will be affected more, and are more unstable, so losing PCNA will have more of an effect on the cancer cells.

The main idea is to do as much damage to the cancer cells while doing as little to normal cells as possible.

Edit: after reading the paper, the drug is targeting a cancer specific isoform of PCNA. This isn’t a mutation, but a slightly different form of the existing protein. Cancers tend to rely on this specific version, while healthy cells can use a different isoform (or both?). A selective inhibitor for this isoform would be cancer cell specific. There would still be some effect on healthy cells, but not as much as with a traditional cytotoxic chemotherapy.