According to the NEJM paper and the Times report, all 12 patients had rectal cancer that had not spread beyond the local area, and their tumors all exhibited a mutation affecting the ability of cells to repair damage to DNA.

After being treated with the drug, dostarlimab, all 12 are now in complete remission, with no surgery or chemotherapy, no severe side effects — and no trace of cancer whatsoever anywhere in their body.

https://www.nbcnewyork.com/news/health/nyc-cancer-trial-delivers-unheard-of-result-complete-remission-for-everyone/3721476/