This invention provides methods of treating androgenetic alopecia (AGA), acne, rosacea, prostate cancer, and benign prostatic hypertrophy (BPH), comprising the step of contacting a subject with a compound or composition capable of decreasing prostaglandin D2 (PGD2) level or activity, a downstream signaling or receptor pathway thereof, or prostaglandin D2 synthase level or activity； methods of stimulating hair growth, comprising the step of contacting a subject with a compound or composition capable of increasing or decreasing the activity or level of a target gene of the present invention, or with a protein product of the target gene or an analogue or mimetic thereof； and methods of testing for AGA and evaluating therapeutic methods thereof, comprising measuring PGD2 levels.
