A 59-year-old man complains of progressive weakness. He reports that his stools are very dark. Physical examination demonstrates fullness in the right lower quadrant. Laboratory studies show iron deficiency anemia, with a serum hemoglobin level of 7.4 g/dL. Stool specimens are positive for occult blood. Colonoscopy discloses an ulcerating lesion of the cecum. Laboratory studies of the surgical specimen obtained from the patient demonstrate hypermethylation of the p53 gene. Which of the following best characterizes this biochemical change in the neoplastic cells?