For histologic analysis, whole skin tissues were fixed with 10% formalin for 24 hrs and then transferred to 70% ethanol for paraffin blocking and staining. The formalin-preserved tissue was sliced with a scalpel blade and arranged in a cassette containing foam biopsy pads and submitted to the UNC Animal Histopathology Core Facility for processing, embedding, sectioning, and Hematoxylin & Eosin (H&E) staining. For TUNEL assays to detect apoptotic cells, mouse skin was collected and formalin-fixed, and paraffin-embedded sections were analyzed with the DeadEnd Fluorometric TUNEL System (G-3250, Promega) to measure the fragmented DNA of apoptotic cells (Wang et al., 2011 (link)). To measure repair of UV-induced (6-4) photoproducts [(6-4) PPs], paraffin-embedded sections were stained with anti-(6-4) PP antibody (Cosmo Bio USA) as described previously (Gaddameedhi et al., 2010 (link)). Fluorescence images were captured using either a Leica inverted DMIRB fluorescence microscope at the UNC Michael Hooker Microscopy Facility or an Olympus IX81 inverted fluorescence microscope at the UNC Microscopy Services Laboratory. For each mouse, up to 25 fields were counted in a blinded manner.