X-Chromosome Inactivation and Autosomal Random Monoallelic Expression as “Faux Amis”

Funding: The work of NK and VMB was funded by iNOVA4Health – UIDB/Multi/04462/2020 and UIDP/Multi/04462/2020, a program financially supported by Fundação para a Ciência e Tecnologia (FCT)/Ministério da Educação e Ciência through national funds, and the FCT grant PTDC/BEX-BCM/5900/2014. CFA-P has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 752806. A-VG was supported by Fundação para a Ciência e Tecnologia (FCT), Portugal, through an assistant research contract (CEECIND/02085/2018) and the project grant PTDC/MEDOUT/4301/2020 IC&DT.X-chromosome inactivation (XCI) and random monoallelic expression of autosomal genes (RMAE) are two paradigms of gene expression regulation where, at the single cell level, genes can be expressed from either the maternal or paternal alleles. X-chromosome inactivation takes place in female marsupial and placental mammals, while RMAE has been described in mammals and also other species. Although the outcome of both processes results in random monoallelic expression and mosaicism at the cellular level, there are many important differences. We provide here a brief sketch of the history behind the discovery of XCI and RMAE. Moreover, we review some of the distinctive features of these two phenomena, with respect to when in development they are established, their roles in dosage compensation and cellular phenotypic diversity, and the molecular mechanisms underlying their initiation and stability.publishersversionpublishe