The genetic mutations in cancer cells may vary in every patient, a study found, suggesting that drugs will need to be tailored more finely to small groups.

The small study, by doctors at the Brigham and Women’s Hospital in Boston and scientists from a gene-reading unit of Roche Holding AG, is among the first to look comprehensively at the genes in cancerous tumors to find which genes went awry. It’s part of a new wave of medical studies using cheaper ways of reading DNA — the chemical blueprint found in every cell — that promise to change the understanding of disease.