If a disease affects men and women differently, does the disease’s mechanism differ by sex? My guess would be no. However, a recent article has me wondering. Schizophrenia symptoms, age of onset, and disease course differ in men and women, and some researchers report increased risk of schizophrenia in men relative to women. Now Shifman et al. report a single nucleotide polymorphism (SNP) associated with schizophrenia in women but not men in a recent article in PLoS Genetics.