NUSAP1 is a nucleolar-spindle-associated protein that plays a role in spindle microtubule organization. The deduced 440-amino acid human protein contains an N-terminal SAP motif, several putative phosphorylation sites, a KEN box, PEST sequence, and a highly charged C-terminal helical domain. The mouse Nusap1 protein contains 427 amino acids and has a calculated molecular mass of 48.6 kD. Northern blot analysis detected a 2.4-kb Nusap1 transcript displaying a 10-fold higher level of expression in differentiating MC3T3E1 cells compared to levels in quiescent cells. Synchronized cells showed highest Nusap1 transcription in late S/G2, and lowest in G1 phase. During metaphase and anaphase, the bundles became more defined, localizing in a 'spikes' pattern around decondensing chromosomes.