Common chromosomal fragile sites appear to be ubiquitous in humans and other mammals, and, although the molecular basis and function of these sites remain an enigma, it has been speculated that they may be a cytogenetic expression of gene activity. A population survey of 28 twin pairs was conducted to assess the heritability of common fragile-site expression. Our data yielded a heritability estimate of .88 for total site expression, suggesting that these sites may result from some common process that is under relatively stringent genetic control. An analysis of the expression of individual autosomal sites revealed that expression on both homologues in the same cell occurred more frequently than expected.