The Xenopus p27(Xic1) gene encodes a cyclin dependent kinase (CDK) inhibitor of the Cip/Kip family. We have previously shown that p27(Xic1) is expressed in the cells of the neural plate as they become post-mitotic (Development 127 (2000) 1303). To investigate whether p27(Xic1) is necessary for cell cycle exit and/or neuronal differentiation, we used antisense morpholino oligos (MO) to knockdown the protein levels in vivo. For such knockdown studies, Xenopus tropicalis is a better model system than Xenopus laevis, since it has a diploid genome. Indeed, while X. laevis has two p27(Xic1) paralogs, p27(Xic1) and p28(Kix1), we have found only one ortholog in X. tropicalis, equidistant from the X. laevis genes. The X. tropicalis p27(Xic1) was expressed in a similar pattern to the X. laevis gene. Depletion of p27(Xic1) in X. tropicalis caused an increase in proliferation and a suppression of the neuronal differentiation marker, N-tubulin. At the same time, we found an increase in the expression of ElrC, a marker of cells as they undergo a transition from proliferation to differentiation. We conclude that p27(Xic1) is necessary for cells to exit the cell cycle and differentiate; in its absence, cells accumulate in a progenitor state. The expression of p27(Xic1) in the embryo is regionalised but the transcriptional regulation of p27(Xic1) is not well understood. We report the isolation of a p27(Xic1) genomic clone and we identify a 5' region capable of driving reporter gene expression specifically in the neural tube and the eye.