The ribonuclease Dicer plays a central role in the microRNA pathway by catalyzing the formation of 19-24-nucleotide (nt) long microRNAs. Subsequently incorporated into Argonaute 2 (Ago2) effector complexes, microRNAs are known to regulate messenger RNA (mRNA) translation. Whether shorter RNA species derived from microRNAs exist and play a role in mRNA regulation remains unknown. Here, we report the serendipitous discovery of a 12-nt long RNA species corresponding to the 5' region of the microRNA let-7, and tentatively termed semi-microRNA, or smiRNA. Using a smiRNA derived from the precursor of miR-223 as a model, we show that 12-nt long smiRNA species are devoid of any direct mRNA regulatory activity, as assessed in a reporter gene activity assay in transfected cultured human cells. However, smiR-223 was found to modulate the ability of the microRNA from which it derives to mediate translational repression or cleavage of reporter mRNAs. Our findings suggest that the 12-nt RNA species, generated along the microRNA pathway, may participate to the control of gene expression by regulating the activity of the related full-length mature microRNA in vivo.