A clinical trial of beta carotene to prevent basal-cell and squamous-cell cancers of the skin. The Skin Cancer Prevention Study Group. - Greenberg,E.R.; Baron,J.A.; Stukel,T.A.; Stevens,M.M.; Mandel,J.S.; Spencer,S.K.; Elias,P.M.; Lowe,N.; Nierenberg,D.W.; Bayrd,G. (N Engl J Med, 1990)