Many of the factors that control branching morphogenesis in the developing lung have been identified but much less is known about how the multipotent endoderm of the ventral foregut becomes committed to a pulmonary fate. On p. 35, Serls and colleagues report that a concentration threshold of fibroblast growth factors(FGFs) produced by the cardiac mesoderm patterns the foregut endoderm into lung and liver. They show that in mouse tissue explants, cardiac mesoderm induces the expression of NKX2.1, the earliest known marker of respiratory epithelium, and of later lung-specific markers in the ventral endoderm. An exogenous source of FGF1 and FGF2 can replace the requirement for cardiac mesoderm: low FGF concentrations activate liver-specific genes; higher concentrations activate lung-specific genes. Finally, other results indicate that signalling through FGFR4 is involved in lung specification.