Mitogen-activated-protein kinase (MAP kinase) cascades are effector mechanisms for many growth factor signals implicated in developmental processes, including appendage outgrowth and organogenesis. The cascade culminates in extracellular-signal-regulated MAP kinase (ERK), which enters the nucleus. ERK activity reflects the competing actions of upstream activator kinases and inhibitory MAP kinase phosphatases. We have studied embryonic expression of the dual-specificity MAP kinase phosphatase PYST1/MKP3, which is a specific and potent regulator of the ERK class of MAP kinases. We found dynamic patterns of mPyst1 messenger RNA in important signalling centres associated with cell proliferation and patterning in developing mouse embryos, including presegmental paraxial mesoderm, limb bud and branchial arch mesenchyme, midbrain/hindbrain isthmus, and nasal, dental, hair, and mammary placodes. Most of these have been characterised as sites of FGF/FGFR signalling.