Transcription factor that binds 5'-AACGG-3' motifs in gene promoters (PubMed:21862669). Transcription activator involved in the regulation of cytokinesis, probably via the activation of several G2/M phase-specific genes transcription (e.g. KNOLLE) (PubMed:17287251, PubMed:21862669). Transcription repressor that regulates organ growth. Binds to the promoters of G2/M-specific genes and to E2F target genes to prevent their expression in post-mitotic cells and to restrict the time window of their expression in proliferating cells (PubMed:26069325). Required for the maintenance of diploidy (PubMed:21862669). Component of a DREAM-like complex which modulates a variety of developmentally regulated genes and of the mitotic genes in proliferating and differentiated cells. A number of isoforms are produced. According to EST sequences. Expressed ubiquitously at low levels (PubMed:10743663). Expressed in roots, cotyledons, flowers and leaves, especially in vascular tissues (PubMed:17287251). Accumulates in the columella root cap. Also present in floral organs in young flower buds. Strongly expressed in vascular tissues of filaments and anthers. Weakly and uniformly present in the developing embryo and maternal tissues (PubMed:17287251). Expressed both in proliferating and maturing stages of leaves (PubMed:26069325). Constant levels during cell cycle. Activated by CYCB1. The double mutant myb3r1 myb3r4 often fails to complete cytokinesis, resulting in multinucleate cells with gapped walls and cell wall stubs in diverse tissues (e.g. in embryo during the first or second division after fertilization, in stomata guard mother cell) and several pleiotropic developmental defects, and associated with the selective reduction of several G2/M phase-specific genes transcript levels (e.g. CYCB2, CDC20.1 and KNOLLE). Hypersensitivity to caffeine, an inhibitor of cytokinesis (PubMed:17287251, PubMed:21862669). In triple mutant myb3r1 myb3r3 myb3r5, up-regulation of many G2/M-specific genes leading to larger seeds, organs and embryos due to overproliferation and ectopic cell divisions (PubMed:26069325).