HMGB1 expression in murine skin and tumors after chronic UV irradiation. SKH-1 mice were exposed three times weekly to 2,240 J/m2 UVB for 15 weeks, after which mice were maintained without UV exposure for an additional 9 weeks (24 weeks total). Age-matched, unirradiated mice served as controls. Representative images of HMGB1 immunostaining are shown for unirradiated skin (a), chronically irradiated skin (b), grade 1 papilloma (c), grade 2 papilloma (d), grade 3 papilloma (e), and microinvasive squamous cell carcinoma (f). Scale bars 100 μm; brown positive staining