ERV3-1 and neoplasm: Mouse dS3 expressed six genes recently discovered to have maternally biased parent-of-origin imprinting51—Adamts5, Erv3, Mfap5, Tfpi2, Tnfrsf11b and Tnfrsf23 (Fig. 3a): these genes are tumour suppressors52,53 and may represent mechanisms to regulate trophoblast invasion.