Abstract
There is a large gap between the aspiration of considering sex as biological variable and the execution of such studies, particularly in genomic studies of human cancer. This represents a lost opportunity to identify sex-specific molecular etiologies that may underpin the dramatic sex differences in cancer incidence and outcome. There are conceptual and practical challenges associated with considering sex as a biological variable, including the definition of sex itself and the need for novel study designs. A better understanding of cancer mechanisms, resulting in improved outcomes, will reward the effort invested in incorporating sex as a biological variable.