By immunohistochemical analysis we also investigated the population of CD15+ cells in tumors of Ptch1+/−/Tis21KO and Ptch1+/−/Tis21WT mice, without finding any difference between the two genotypes in the total number of tumor stem cells or in their proliferating or apoptotic fractions (data not shown). This evidence concerns the gene PTCH1 and neoplasm.