Fu et al. bred transgenic mice to study the role of retinoblastoma 1 in neural-crest-derived melanocytes. “TyrCre+;Rb1fl/fl mice exhibited no melanocyte defects but died unexpectedly early with intestinal obstruction, striking defects in the enteric nervous system (ENS), and abnormal intestinal motility,” say the authors. Mutant mice had twice as many enteric glia as control mice. Myenteric neuron number was normal, but DNA replication without cell division caused NO-producing myenteric neurons to develop very large nuclei.