Abnormal proteins in neoplastic cells are present in a variety of animal tumor systems. Our laboratory has isolated several of these abnormal proteins from murine malignant melanoma by preparative gel electrophoresis; these proteins are similar in many respects to analogous proteins found in normal tissues. Examination of these proteins by polyacrylamide gel electrophoresis, amino acid analysis, carboxy and amino terminal analysis and peptide mapping after cyanogen bromide cleavage, supports the theory that these abnormal proteins are the result of deleted sequences in the peptide chain during the attempted synthesis of normal proteins in neoplastic tissue.