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The present study aimed to investigate the mechanism underlying the effect of adzuki bean extract on the differentiation of 3T3-L1 adipocytes. Treatment of these adipocytes with the extract significantly suppressed the expressions of the Pparγ, C/ebpα, and Fabp4 genes that are involved in adipogenesis (adipocyte differentiation). Oil Red O staining revealed that fat accumulation within these adipocytes is reduced significantly when the cells are treated with the extract. To explore the impact of adzuki bean extract on the differentiation of 3T3-L1 adipocytes, we investigated the effect of adzuki bean extract on the expression of cycle-related factors. We observed reduced expression of cyclin D1 mRNA and phosphorylated Rb (pRb), a downstream factor of cyclin D1 at 24 h after the application of a differentiation-inducing stimulus. Meanwhile, at 72 h after the stimulation, the expression of pRb increased with an increase in the amount of adzuki bean extract. The proliferation of cells declined significantly on treatment with the extract at 16 h after the stimulation but accelerated significantly at 72 h after the stimulation. These results suggest that adzuki bean extract contains an active ingredient that suppresses the differentiation of adipocytes. As a possible mechanism, the extract may alter the mitotic clonal expansion (MCE) which is essential for adipogenesis.