X chromosome inactivation in mammalian females occurs early in embryonic development and renders most genes on the inactive X chromosome transcriptionally silenced. As a consequence, females will display an X chromosomal parent-of-origin mosaiscism with regard to which parental allele that is expressed. Some genes, however, escape inactivation and will therefore be expressed from both alleles. In this study, we have investigated if the X-linked MAO-A gene has bi- or mono-allelic expression. This information would indicate whether or not MAO-A gene dosage could potentially explain the observed gender differences that show functional connections to the serotonin system, such as aggression, and impulsiveness. To investigate the X inactivation status of MAO-A we have used primary clonal cell cultures, on which allelic expression was assessed with RFLP analysis. Our results show that the MAO-A gene has mono-allelic expression in these cells. This could have important implications for understanding traits that display gender differences. (c) 2006 Elsevier Inc. All rights reserved.