Model of the genetic interaction between Dlk1 and Grb10 . Normal growth is observed only in wild type animals, with intact Grb10 and Dlk1 genes. Gene knockout (indicated by an ‘X’ over the relevant genotypes) of maternal Grb1O (Grb10 m/+mice) results in overgrowth, whereas knockout of paternal Dlk1 (Dlk1 +/p mice) results in growth restriction. Simultaneous knockout of both genes (Grb10 m/+/Dlk1 +/p mice) results in overgrowth, with associated changes in body proportions, histology and physiology, essentially as in Grb10 m/+ mice. Thus, Grb10 is an inhibitor of fetal growth while Dlk1 acts to promote growth by inhibition of Grb10.