Philip Leder, a biologist who helped decipher the genetic code and discovered a genetic cause of cancer, died on Feb. 2 at his home in Chestnut Hill, Mass. He was 85.

The cause was complications of Parkinson’s disease, his son Benjamin Z. Leder said.

Dr. Leder helped accomplish the final step in deciphering the genetic code early in his career. In immunology, he went on to help unravel the genetic mechanisms behind the great diversity of antibody molecules. He then discovered that the misregulation of a gene that guides the growth of cells was a major cause of cancer.

Dr. Leder was a postdoctoral student at the National Institutes of Health in Bethesda, Md., in 1962 when — nine years after the structure of DNA was discovered — he was recruited by Marshall Nirenberg. The two men entered a furious race by biologists around the world to solve the genetic code, the means whereby successive units of DNA specify the order of the amino acids, the building blocks of protein.

Amid much theoretical speculation about how this might work, Dr. Nirenberg chanced upon a practical approach to the problem: feeding synthetic strands of RNA — DNA’s messaging system — to living cells and seeing which amino acids were summoned to the protein-making machinery.