Aggregates keratin intermediate filaments and promotes disulfide-bond formation among the intermediate filaments during terminal differentiation of mammalian epidermis. In the stratum granulosum of the epidermis, localized within keratohyalin granules (PubMed:1429717). In granular keratinocytes and in lower corneocytes, colocalizes with calpain-1/CAPN1 (PubMed:21531719). Expressed in skin, thymus, stomach, tonsils, testis, placenta, kidney, pancreas, mammary gland, bladder, thyroid, salivary gland and trachea, but not detected in heart, brain, liver, lung, bone marrow, small intestine, spleen, prostate, colon, or adrenal gland (PubMed:19384417). In the skin, mainly expressed in stratum granulosum of the epidermis (PubMed:1429717) (PubMed:19384417). In cultured foreskin fibroblasts, up-regulated in response to Ca(2+) stimulation. Filaggrin is initially synthesized as a large, insoluble, highly phosphorylated precursor containing many tandem copies of 324 AA, which are not separated by large linker sequences. During terminal differentiation it is dephosphorylated and proteolytically cleaved. The N-terminal of the mature protein is heterogeneous, and is blocked by the formation of pyroglutamate. Undergoes deimination of some arginine residues (citrullination). The disease is caused by variants affecting the gene represented in this entry. Disease susceptibility is associated with variants affecting the gene represented in this entry. Belongs to the S100-fused protein family.