Represses the transposition of Tc1, Tc3, Tc4, and Tc5, perhaps by degrading transposon-specific messages. Also affects sperm development, sensitivity to RNAi of mainly germline expressed genes, silencing of some germline transgenes, X chromosome loss, and is required for cosuppression (functional silencing of chromosomal loci induced by transgenes) and for silencing induced by antisense RNA oligomers. Belongs to the mut-7 family.