Ectopically expressed Airn lncRNA deposits Polycomb with a potency that rivals Xist .

We report that when expressed at similar levels from an isogenic locus, the lncRNA induces Polycomb deposition with a potency that rivals . However, when subject to the same degree of promoter activation, is more abundant and more potent than . Our data definitively demonstrate that the lncRNA is functional and suggest that achieved extreme potency in part by evolving mechanisms to promote its own abundance.