A 26-year-old man with a family history of colon carci noma undergoes a surveillance colonoscopy.
It reveals hun dreds of polyps in the colon, and two focal 0.5-cm ulcerated areas.
A biopsy specimen from an ulcer reveals irregularly shaped glands that have penetrated into the muscular layer.
Which of the following molecular events is believed to occur very early in the evolution of his colonic disease process?
Loss of heterozygosity affecting the p53 gene