This is a Phase 2b, double-blind, randomized, placebo-controlled study to evaluate the safety and efficacy of one or two applications of topically applied GAM501 (Ad5PDGF-B/Bovine Type I Collagen Gel) in subjects ≥ 18 years old with non-healing diabetic foot ulcers. Approximately 210 adult subjects with Type I or Type II diabetes mellitus will be enrolled at approximately 30 investigational sites in the United States.