The transcription factor Mesp is essential for early heart development in chordates from sea squirts (Ciona) to vertebrates. But how it is regulated in the emerging heart field and how it functions remain unclear. On p. 4811,Davidson and colleagues report that, in Ciona, Mesp gene expression is controlled by the T-box transcription factor Tbx6c and that its primary function is heart specification. The researchers define a minimal cardiac enhancer for Mesp in two species of Ciona through which Tbx6c activates Mesp expression. Expression of constitutively active Mesp under the control of this enhancer results in ectopic heart differentiation in the anterior tail – heart progenitor cells normally leave the tail region of the early embryo before differentiating. Thus, the researchers conclude, Mesp acts as a cardiac determinant independently of any role in heart cell migration in Ciona and probably also in vertebrates.