['ERK5 is a member of the mitogen activated protein kinase family activated by certain mitogenic or stressful stimuli in cells, but whose physiological role is largely unclear.', 'To help determine the function of ERK5 we have used gene targeting to inactivate this gene in mice. Here we report that ERK5 knockout mice die at approximately E10.5. In situ hybridisation for ERK5, and its upstream activator MKK5, showed strong expression in the head and trunk of the embryo at this stage of development. Between E9.5 and E10.5, multiple developmental problems are seen in the ERK5-/- embryos, including an increase in apoptosis in the cephalic mesenchyme tissue, abnormalities in the hind gut, as well as problems in vascular remodelling, cardiac development and placental defects.'] Does knockout of ERK5 cause multiple defects in placental and embryonic development?