Reduce the expression of some gene product (usually a protein), typically by using RNA interference (RNAi) to target destruction machinery (Ago and friends) to the mRNA instructions for making that protein. Knock-down thus reduces the levels of the mRNA and therefore the protein, but it doesn’t touch the actual gene (i.e. it’s not gene editing). Knock-down is often done using siRNA (small interfering RNA) or shRNA (short hairpin RNA). Unlike knock-out, the effects of knock-down are partial and temporary. Much more in the linked-to post.
