MAOA appears to be among the 15% of X-linked genes that escape X-inactivation (Carrel and Willard, 2005). However, partial X inactivation could conceivably result in higher brain MAOA expression levels for female homozygotes than male hemizygotes. Furthermore, one study has shown epigenetic gene regulation in the MAOA promoter region only in females, representing a possible dosage compensation mechanism that does not correlate with X-inactivation (Pinsonneault et al, 2006). This may be another explanation for why our findings were more striking in pre-pubertal girls than boys.
