During embryogenesis, genomic imprinting ensures that only one parental allele of certain genes is subsequently expressed. One of the first imprinted genes to be identified was the H19 gene, but the function of the non-coding RNA produced from this gene has remained a mystery. Now, Gabory and colleagues report that H19 RNA is a trans regulator of the imprinted gene network (IGN), a group of co-expressed genes that includes H19and that controls fetal and early postnatal growth in mice (see p. 3413). The researchers report that postnatal growth is reduced in two transgenic mouse lines that overexpress H19 and that the expression of Igf2(also a member of the IGN) is reduced in the transgenic embryos. Conversely, H19 deletion leads to an upregulation of several members of the IGN that can be reversed by transgenic expression of H19. This first in vivo evidence for a functional role for H19 RNA provides new insights into how genomic imprinting helps to control embryonic growth.