A   model  for  the  multistep  development  of  familial  adenomatous
   polyposis (FAP).
   The  first  step  is  the  mutantion  of one allele of the APC gene on
   chromosome  5.  Other mutations involving genes on chromosomes 12, 17,
   and   18  in  cells  of  benign  adenomas  can  lead  to  a  malignant
   transformation that results in colon cancer.