There are exceptions to the two-hit rule for tumor suppressors, such as certain mutations in the p53 gene product. p53 mutations can function as a dominant negative, meaning that a mutated p53 protein can prevent the function of the natural protein produced from the non-mutated allele. Other tumor-suppressor genes that do not follow the two-hit rule are those that exhibit haploinsufficiency, including PTCH in medulloblastoma and NF1 in neurofibroma. Another example is p27, a cell-cycle inhibitor, that when one allele is mutated causes increased carcinogen susceptibility.