Kaplan-Meier survival curves of oral cavity SCC patients grouped according to Smad expression. Patients were categorized as positive (above) or negative (equal or below) according to median Smad expression in tumors. Log Rank test was performed for curves comparison. In E and F, patients were grouped according to the co-expression of Smad2 and Smad6. Smad2+/Smad6-: n = 13; Smad2+/Smad6+: n = 11; Smad2-/Smad6-: n = 13; Smad2-/Smad6+: n = 11.