Tessa and Alex have published a paper "Vg1-Nodal heterodimers are the endogenous inducers of mesendoderm" in eLife.
One of the major steps in development is the formation of the germ layers: endoderm, which will form the internal organs, mesoderm, which will form the muscles and bones, and ectoderm, which will form the skin and nervous system. The Nodal signaling pathway is required to generate the endoderm and mesoderm, and according to current models, Nodal is the ligand that activates the pathway.
In this study, Tessa and Alex found, instead, that the active ligand is this pathway is a heterodimer of Nodal and Vg1. Vg1 has a similar structure as Nodal, but its function has been a mystery for 30 years.