HSC-2, CAL-27 and SCC-4 cell lines were purchased from ATCC and were cultured in DMEM (Invitrogen; Thermo Fisher Scientific, Inc.) containing with 10% fetal bovine serum at 37°C in a 5% CO2 incubator. The normal human oral keratinocyte (NHOK) cells which served as control cells were obtained from the ScienCell Research Laboratories, Inc. (cat. no. 2610) and cultured as previously described (15 (link)).