The small GTPase Rho, which regulates cell shape, is thought to contribute to cytokinesis. Recently, Citron was characterized as a Rho target. This large protein contains a Ser/Thr kinase domain related to that of ROCK, another Rho effector. Both endogenous Citron and recombinant Citron localize to the cleavage furrow in dividing cells and to the midbody in post‐mitotic cells. Moreover, overexpression of Citron deleted from its C‐terminal sequence caused abnormal contractions specifically during cytokinesis, resulting in the formation of multinucleated cells. Cell shape, F‐actin, intermediate filaments, and microtubules appeared essentially normal in these cells during interphase. Thus, Citron is a Rho effector that appears to function during cytokinesis, modulating its contractile process. In brain, however, Citron is highly expressed in a subset of neurons as a brain‐specific isoform that lacks a kinase domain, Citron‐N. This protein accumulates in synapses and associates to the NMDA receptor via interaction with the adaptor protein PSD95, suggesting that the function of Citron is specialized in the neurons. Microsc. Res. Tech. 49:123–126, 2000. © 2000 Wiley‐Liss, Inc.