To determine if MLH1 plays a role in processing psoralen-crosslinked DNA in human cells, we performed a cell viability assay following DNA damage induced by PUVA [cells were exposed to the psoralen derivative, 4′-hydroxymethyl-4,5′,8-trimethylpsoralen (HMT), at concentrations ranging from 10−8 to 10−5 M, and UVA irradiated at 1.8 J/cm2] in MLH1-proficient (A2780) and isogenic MLH1-deficient (A2780/cp70) ovarian cancer cells. This evidence concerns the gene MLH1 and ovarian cancer.