Polypeptide(s) (I), consisting of the chitinase family protein 'protein YKL-40' (Ia) (isolated from human chondrocyte cultures) and its derivatives and analogs, are used for (a) cosmetic stimulation of terminal epithelial differentiation and/or epithelial proteoglycan (EPG) synthesis or (b) therapeutic stimulation of terminal epithelial differentiation, EPG synthesis and/or epithelial antimicrobial defenses. The use of specific polypeptide(s) (I), nucleic acids (II) encoding (I) or agents (III) modulating the expression of (I), the activity of (I) and/or the release of (I) from epithelial proteoglycan (EPG) matrix is claimed for (a) cosmetic stimulation of terminal epithelial differentiation and/or EPG synthesis or (b) production of a therapeutic composition for stimulation of terminal epithelial differentiation, EPG synthesis and/or epithelial antimicrobial defenses. (I) are (i) polypeptides encoded by all or part of a nucleotide sequence given in the specification as SEQ ID NO: 1 (1152 bases), SEQ ID NO: 2 (1089 bases) or SEQ ID NO: 1 (63 bases) or (ii) analogs or fragments of polypeptides as in (i). Independent claims are included for: (1) the use of (I) as a tool for screening biological or chemical compounds (III); (2) the use of (I) or (II) as a tool for characterizing an epithelial condition, specifically selected from desquamation, ichthyosis, hyperkeratosis, epidermal drying, aging or photoaging; (3) a method for characterizing an epithelial condition, involving (a) determining the content of (I) or (II) in an epithelial sample and (b) comparing the obtained value with a reference value; and (4) a method for screening antiaging agents, involving (a) containing (I) with test compound(s) under suitable conditions for demonstrating the biological activity of (I) and (b) determining the biological activity of (I). ACTIVITY : Dermatological; Antiinflammatory; Antibacterial; Fungicide; Vulnerary; Antipsoriatic; Cytostatic; Antipruritic; Immunomodulator; Antiseborrheic; Depilatory. MECHANISM OF ACTION : Stimulation of terminal epithelial differentiation; Stimulation of epithelial proteoglycan synthesis.