P4HA3 and achondroplasia: The potential importance of understanding allele-specific gene silencing is underscored for three of the genes we have identified, Agc1, Thrsp and P4ha3. Proof of monoallelic expression for these genes in human tissue would constitute a separate study, but it is worth noting that in mice, mutation of Agc1 leads to a lethal phenotype at birth, and that there is a possible link to the human genetic disorder achondroplasia [39].