Concept: What RNAi targets

RNA interference works by using double‑stranded RNA whose sequence is complementary to the mature mRNA of the target gene.
Therefore, effective RNAi constructs are built from any region that actually appears in the processed mRNA (5′ UTR, coding exons, 3′ UTR), but not from introns, which are removed during splicing.

Option‑wise explanation

1. 5′ UTR of the mature transcript

   • The 5′ untranslated region is part of the mature mRNA.

   • A dsRNA/siRNA designed against this region can base‑pair with the transcript and trigger its degradation or translational repression.

   • Hence, it can be used for RNAi constructs.

2. 3′ UTR of the mature transcript

   • The 3′ UTR is also present in the mature mRNA and is a common regulatory target (many endogenous miRNAs bind here).

   • RNAi constructs frequently use 3′ UTR sequences to achieve specific knockdown.

   • So this region is also suitable.

3. Exonic region

   • Exons make up the coding sequence (and sometimes UTR parts) of the mature mRNA.

   • Most synthetic siRNAs/shRNAs are designed against coding exons to ensure strong, specific silencing.

   • Therefore, exonic regions are standard RNAi targets.

4. Intronic region – NOT used (correct choice)

   • Introns are removed during pre‑mRNA splicing and are absent from the mature cytoplasmic mRNA.

   • An RNAi construct derived from intronic sequence would not find a complementary region in the mature transcript, so it would not efficiently knock down gene expression.

   • Thus, intronic regions are not typically used for designing RNAi constructs.

So, the region not used for making an RNAi construct to knock down expression of a gene is the intronic region (option 4).