tumor suppressor genes make up another group of genes that regulate cell division. genes from this group produce proteins that signal cells when they are getting too crowded, proteins whose function is to repair dna, and still other proteins that regulate apoptosis (pre-programmed cell death). a tumor suppressor gene called p53 causes apoptosis when the cell is worn out or when defects are detected. where in the cell cycle would a tumor suppressor gene repair dna?