Silencing of specific mRNA is possible via RNAi because of

RNA interference (RNAi) works by introducing a double‑stranded RNA (dsRNA) that is complementary to the target messenger RNA (mRNA). The dsRNA is recognized by the cellular enzyme Dicer, which cuts it into short interfering RNAs (siRNAs) of about 21–23 nucleotides.

The siRNA strands are then incorporated into the RNA‑induced silencing complex (RISC). Within RISC, the guide strand of the siRNA pairs perfectly with the target mRNA because of its sequence complementarity. This base‑pairing directs RISC to cleave the mRNA, preventing it from being translated into protein.

Therefore, the specific silencing of a particular mRNA is possible only when the introduced RNA is a complementary double‑stranded RNA (dsRNA), which provides the necessary sequence matching to the mRNA for the RNAi machinery to recognize and degrade it.