Cystatin A (acid cysteine proteinase inhibitor, ACPI) is a natural inhibitor of cysteine proteinases and is located mainly in the keratohyaline granules of the stratum granulosum and the cornified envelope of the stratum corneum in the epidermis. Human cystatin A consists of 98 amino acid residues and belongs to cystatin subfamily I (stefins). Among the target enzymes inhibited by cystatin A are cathepsins B, C, H, L, and S, papain and actinidin. Cystatin A has also been detected in infiltrative breast carcinoma.