Frederick Griffith discovered in 1928 that heat-killed bacteria contained a transforming factor that could change harmless bacteria into disease-causing bacteria. Oswald Avery and colleagues in 1944 determined that this transforming factor was DNA. Rosalind Franklin's x-ray crystallography work in 1952 provided evidence that DNA has a helical structure, which James Watson and Francis Crick used to propose their double helix model of DNA structure in 1953.