Twenty years ago, researchers discovered that the mRNA for Vg1, a transforming growth factor β family member, localizes to the vegetal cytoplasm of Xenopus oocytes, making Vg1 a candidate for the mesoderm-inducing signal released by vegetal cells. However, its role in vivo has remained mysterious until now. On p. 15, Birsoy and colleagues now show that maternal Vg1 is an essentially required signalling molecule during Xenopus development. They report that gastrulation is delayed, and that anterior and dorsal development is reduced, in embryos depleted of Vg1 using an antisense oligonucleotide. The mystery of the role of Vg1 in vivo arose, they explain, because the original Vg1 clone encodes proline at position 20. This means it is inefficiently processed to active Vg1 in vivo and so fails to rescue Vg1-depleted embryos. By showing that a Vg1 allele with serine at this position rescues Vg1-depleted embryos, the researchers demonstrate conclusively that Vg1 is an essential maternal regulator of embryonic patterning.