Cysteine protease inhibitor which inhibits members of the peptidase C1 family (PubMed:12704112, PubMed:15664654). Does not inhibit asparaginyl endopeptidase (PubMed:15664654). Required for the uptake and/or processing of yolk proteins during the development of oocytes, probably by regulating the catalytic activity of cysteine proteases cpl-1 and cpz-1 (PubMed:16857685). May play a protective role against exogenous cysteine proteases derived from soil bacteria or fungi, or rotting fruits and vegetation (PubMed:24001183). Localizes to the sheath cell cytoplasm surrounding germ cells and oocytes. Localizes to yolk granules in the developing oocyte. Expressed in germ cells, developing oocytes and sperm, sheath cells surrounding germ cells and oocytes and in the eggshell (at protein level) (PubMed:16857685). Expressed in the pharyngeal gland, pharyngeal muscles and some hypodermal cells in the tail (PubMed:16857685). Expressed in embryos, larvae and adults (at protein level) (PubMed:15664654, PubMed:16857685). Expressed in the ecdysed cuticle during molting (at protein level) (PubMed:16857685). In embryos and larvae, expressed in intestinal, hypodermal and pharyngeal cells (PubMed:16857685). Expression transiently increases during early embryonic stages and prior to the larval L1/L2, L2/L3, L3/L4 and L4/adult molts (PubMed:16857685). Belongs to the cystatin family.