A full length N-ras gene has been cloned from both the human fibrosarcoma cell line HT1080 and from normal human DNA. N-ras isolated from HT1080 will efficiently induce morphological transformation of NIH/3T3 cells in a transfection assay, whereas N-ras isolated from normal human DNA has no effect on NIH/3T3 cells. The coding regions of the normal N-ras gene have been sequenced and the predicted amino acid sequence of the N-ras product is very similar to that of the c-Ha-ras1 and c-Ki-ras2 products. By making chimeric molecules between the two cloned genes the activating alteration in the HT1080 N-ras gene has been localised to a single base change that results in an amino acid alteration at position 61 of the p21 N-ras product.