Silencing of gene expression by RNAi in a nematode. Mex-3 protein is a regulator that is expressed and functions early in nematode development. Four early embryos are shown at identical stages of development, with in situ hybridization used to detect the presence of mex-3 transcripts. (a) No staining is seen in a control, with no hybridization probe added. (b) Staining reveals the normal pattern of mex-3 expression in the embryo. (c) Injecting an embryo with an RNA complementary (antisense) to mex-3 mRNA reduces gene expression somewhat. (d) Injecting an embryo with double-stranded RNA corresponding to mex-3 reduces gene expression dramatically.