Early vertebrate embryos are bilaterally symmetrical but mature internal organs show consistent left-right (LR) asymmetry. The establishment of LR positional information has been well studied, but how the organ primordia interpret this information during asymmetric morphogenesis is unclear. Hecksher-Sørensen and colleagues identify a specialised columnar epithelium – the splanchnic mesodermal plate (SMP) – as the organiser for the leftward growth of the spleno-pancreatic region under the control of the LR asymmetry pathway in mice (see p. 4665). They report that the asplenic mutant dominant hemimelia (Dh) lacks the SMP, an initially bilateral structure surrounding the gut endoderm. Another asplenic mutant carrying a Bapx1-targeted disruption has a defective SMP, indicating that Bapx1 regulates the SMP functions that are required for spleno-pancreatic laterality. Given that the Bapx1 and Dh mutants have normal hearts, lungs and livers, the researchers suggest that LR positional information is interpreted differently by each organ primordia.