Throughout development, a proper balance between the proliferation and differentiation of progenitor cells is essential but the gene regulatory networks that control this balance are only partly understood. Here, Edward Morrisey and colleagues report that miR302/367 (a microRNA cluster) regulates the behaviour of endoderm progenitor cells during mouse lung development (see p. 1235). MicroRNAs (short RNA molecules that silence complementary target mRNA sequences) are expressed in clusters from a single primary transcript. The researchers show that, in early lung endoderm, the miR302/367 cluster is a target of the transcription factor Gata6, which is known to regulate lung endoderm progenitor differentiation and proliferation. Increased or decreased miR302/367 expression, they report, alters the balance of lung endoderm progenitor differentiation and proliferation in part through regulation of the tumour suppressor Rbl2 and the cell-cycle regulator Cdkn1a. Notably, altered miR302/367 expression also disrupts apical-basal polarity of endoderm progenitor cells. Thus, the researchers conclude, miR302/367 directs mouse lung development by regulating multiple aspects of lung endoderm progenitor cell behaviour.