Abzhanov and Tabin found that when the gene for a protein called BMP4 is activated (scientists use the word "expressed") in the growing jaw of a finch embryo, it makes the beak deeper and wider.

In addition to transcriptional regulation of GDNF, a further level of control seems to involve the signalling molecule bone morphogenetic protein (BMP4) and its inhibitor, gremlin 1 (GREM1).