Mesoderm specification in early Xenopus embryos involves several TGFβ family members, including zygotically expressed nodal-related (Xnr)proteins 1-6 and maternally inherited Vg1. On p. 591, Birsoy et al. report that maternal XPACE4, a pro-protein convertase enzyme, plays an important role in embryonic patterning by regulating the production of a subset of mature TGFβ proteins during Xenopus development. The researchers show that XPACE4 is stored as maternal mRNA in the mitochondrial cloud and vegetal hemisphere of the oocyte, and that XPACE4 activity is required for the endogenous mesoderm-inducing activity of vegetal cells before gastrulation. The researchers also provide a comprehensive survey of the effect of XPACE4 depletion on the cleavage spectra of seven TGFβs -Xnr1-3, Vg1, Xnr5, Derrière and ActivinB - present in embryo lysates and blastocoel fluids. They report that XPACE4 cleaves Xnr1-3 and Vg1 but not Xnr5, Derrière or ActivinB.