Mitotic recombination can explain the apparent polyclonal origin of some tumors

Although most human tumors appear to be monoclonal in origin, a few express two G6PD types; on this basis they have been thought to be polyclonal in origin and exceptions to the general rule of monoclonality. In light of the recent discovery that mitotic recombination can cause a shift from genetic heterozygosity to homozygosity in tumor cells, we suggest that such a mechanism can also explain the occasional occurrence of two G6PD types in a monoclonal tumor.