In Drosophila, the NK homeobox gene tinman (tin)is essential for the specification of cardiac progenitors in the early dorsal mesoderm. Like its vertebrate counterpart Nkx2.5, tin is also expressed during cardiac maturation and differentiation. However, its later role in cardiac development is unclear because tin-null embryos have no dorsal vessel (the Drosophila equivalent of a heart), and die. Zaffran et al. now reveal that tin controls the diversification and differentiation of myocardial cells during the later stages of cardiogenesis,through regulatory interactions with Dorsocross and other cardiogenic factors (see p. 4073). The researchers made their discovery by making transgenic fly lines that expressed tin normally during early heart development, but that did not express tin in dorsal vessel cardioblasts at later stages. The dorsal vessel formed in the resulting embryos and was present in surviving adult flies, but myocardial diversification, differentiation and remodelling was defective. These findings provide new information about the molecular pathways that act at later stages of fly, and perhaps also mammalian, heart development.