To identify good model systems for studying the functions of 5α-reductase isoenzymes, we evaluated the mRNA levels of the 5α-reductase isoenzymes in different prostate cell lines, including PWR-1E immortalized normal prostatic epithelial cells; BPH-1-AR benign prostatic hyperplasia (BPH) cells, which stably express AR; LAPC-4 and LNCaP androgen-sensitive prostate cancer cells; and C4-2B4 androgen-independent cells. The gene discussed is AR; the disease is benign prostatic hyperplasia.