The A431 epidermoid carcinoma cell line (ECACC 85090402) was purchased from Merck Life Sciences (Merck Life Science). The HaCaT non-tumorigenic keratinocyte cell line was used as a control and was acquired from the Experimental Zooprophylactic Institute of Lombardia and Emilia Romagna (Brescia, Italy). The HaCaT cell line can be used as non-tumor control of the A431 cells (56 (link)-60 (link)) and it is considered a reliable model for skin diseases and for in vitro carcinogenesis of human skin keratinocytes (61 (link),62 (link)). The HaCaT and A431 cell lines were cultured under the same culture conditions to prevent differences in phototoxicity related to different growth mediums. The cell lines were cultured in high-glucose DMEM (Euroclone S.p.A.) supplemented with 10% fetal bovine serum (FBS; Euroclone S.p.A.) and 1% penicillin/streptomycin antibiotics (Euroclone S.p.A.) at 37°C in an atmosphere containing 5% CO2. Both cell lines were sub-cultured every 3-4 days. Both cell lines were mycoplasma-free.