Schenk JM, Riboli E, Chatterjee N, Leitzmann MF, Ahn J, Albanes D, Reding DJ, Wang Y, Friesen MD, Hayes RB, Peters U. Serum retinol and prostate cancer risk: a nested case-control study in the prostate, lung, colorectal, and ovarian cancer screening trial. Cancer Epidemiol Biomarkers Prev. 2009 Apr;18(4):1227-31.