Specifically recognizes and binds methylated 'Lys-9' of histone H3 (H3K9me), with highest preference for trimethylated 'Lys-9' (H3K9me3) followed by dimethylated 'Lys-9' (H3K9me2) followed by monomethylated 'Lys-9' (H3K9me1) (PubMed:24685137). Plays a role in maintaining correct unc-4 expression in the VC motor neurons where unc-4 is expressed in the vulval but not in the non-vulval VC neurons (PubMed:24348272). Colocalizes with H3K9me2/H3K9me3 in germline nuclei. Expressed in every cell of the embryo (at protein level). In adults, expressed predominantly in the head region and the germline. Expression of unc-4 in all 6 VC motor neurons in contrast to wild-type expression which is only detected in the vulval VC neurons, leading to hyperinhibition of HSN neuron activity and egg-laying defects (PubMed:24348272). RNAi-mediated knockdown in an spr-5 mutant background suppresses progressive sterility and prevents accumulation of histone H3 'Lys-4' dimethylation (PubMed:24685137).