A study of X-inactivation using human-rodent somatic cell hybrids reported that MAOA among several other genes on the X chromosome escapes inactivation (Carrel & Willard, 2005). However, other studies report that MAOA is subject to random X-inactivation. Benjamin et al. (2000) reported non-skewed patterns of inactivation in genomic DNA obtained from blood samples. A study of monozygotic female twins described non-skewed inactivation in a majority (85%) of samples (Fraga et al. 2005), supporting random X-inactivation. Another study of allelic expression of a single nucleotide polymorphism in exon 6 of MAOA in human skin fibroblasts also demonstrated random monoallelic expression (Nordquist & Oreland, 2006).
