MicroRNAs (miRNAs) are small noncoding RNA and play a role in gene expression regulation by inhibiting translation of their target messenger RNAs (mRNAs). In this study, we investigated the effects of endogenous let-7 miRNA on the expression of target genes in various mammalian cells by means of two types of reporter plasmids possessing target sequences for let-7: one carries perfectly matched target sequence for let-7 in the 3'-untranslated region of the luciferase reporter gene to monitor RNA interference (RNAi) activity and the other has three bulged binding sites for let-7 to monitor translation-inhibition activity. The results indicate that different cells have different levels of gene silencing against the target reporter genes. The data presented here suggest that not only microRNA level but also target transcript level likely participate in the generation of a variety of gene silencing.