Nonmuscle myosin IIs (NMM II-B) are involved in basic cellular functions, such as cell motility and cytokinesis. Since most cells express more than one nonmuscle myosin II heavy chain (NMHC) isoform, ablation of a single isoform does not result in abnormal cytokinesis. One exception is cardiac myocytes, which contain only NMHC II-B. Previous work showed that cardiac myocytes from embryonic day 12 (E12) NMHC B-/B- mice develop hypertrophy and binucleation, which are consistent with a defect in cytokinesis (Takeda et al., Mol. Biol. Cell 11, 541a, 2000). To investigate a defect in DNA synthesis in B-/B- cardiac myocytes, we analyzed 5-bromo-2'-deoxyuridine (BrdU) incorporation in E14 B-/B-, B+/B- and B+/B+ mouse hearts and other tissues. BrdU was injected intraperitoneally into pregnant B+/B- intercrossed mice and, after 1 hour, mothers were sacrificed and E14 embryos removed for sectioning and analysis by dual immunofluorescent staining for BrdU and desmin (to label cardiac myocytes). The BrdU index (% of BrdU-positive myocytes) in E14 B-/B- cardiac myocytes was significantly lower than in E14 B+/B+ and B+/B- cardiac myocytes: 13.96 plus or minus 3.21 (n=3, mean plus or minus SD) vs 23.66 plus or minus 3.08 (n=4, mean plus or minus SD), B-/B- vs B+/B+, B+/B- controls, p less than 0.01. The BrdU index of non-cardiac cells in E14 B-/B- heart, which, in contrast to cardiac myocytes, do express NMHC II-A, did not show a significant difference from that of B+/B+ and B+/B- mice: 14.67 plus or minus 4.27 (n=3, mean plus or minus SD) vs 18.23 plus or minus 3.56 (n=4, mean plus or minus SD), B-/B- vs B+/B+, B+/B- controls. The BrdU index of B-/B- liver cells, which also express NMHC II-A, showed no difference in BrdU uptake compared to B+/B+ and B+/B- mice: 45.01 plus or minus 5.93 (n=3, mean plus or minus SD) vs 49.50 plus or minus 4.97 (n=4, mean plus or minus SD). Taken together, BrdU incorporation was decreased specifically in E14 B-/B- cardiac myocytes. Interestingly, we also detected upregulation of cyclin D3 in the E14 B-/B- heart (2-3-fold increase compared to B+/- mice) and preliminary detected upregulation of cyclin D1 and cyclin D2 and cyclin-dependent kinase 4 (cdk4). We conclude that the absence of nonmuscle myosin II-A and II-B in cardiac myocytes results in abnormalities in cytokinesis which are also reflected in other phases of the cell cycle including S (BrdU uptake) and G1 (cyclin D elevation).