The present invention concerns a polypeptide comprising the N-terminal fragment of a cystatin, and/or a cystatin expression inducer for use for the treatment of sterility and/or infertility, and a method of diagnosing and/or predicting sterility and/or infertility of a subject and/or of a couple of subjects, which method comprises measuring the level of at least one cystatin in a biological sample of a male subject and/or measuring the level of at least one cystatin in a biological sample of a female subject, and optionally measuring further the level of at least one cathepsin in a biological sample of said male of female subject.
