Guanine deaminase (GDA) was mainly expressed in human keratinocytes (KCs) and upregulated when treated with a mixture of inflammatory cytokines. (A) The relative GDA mRNA expression levels in normal human melanocytes (NHMs), human dermal fibroblasts (HDFs), and normal human keratinocytes (NHKs) were calculated after normalization to the RPLP0 gene. (B) NHMs cocultured with NHKs were transfected with negative control siRNA (siNC) or GDA (siGDA) for 3 d. To determine whether GDA expression level was decreased and to identify the GDA-expressing cells, the cells were double-stained with GDA (green) and Melan-A (red) and counterstained with 4′,6-diamidino-2-phenylindole (DAPI) (blue). (C) Pigmentary changes in cocultured cells by siGDA were demonstrated in bright-field and phase contrast images. (D) NHKs were stimulated with 4 ng/mL interferon gamma (IFN-γ), 20 ng/mL tumor necrosis factor alpha (TNF-α), and 20 ng/mL interleukin 1 alpha (IL-1α) for 24 h, and the GDA and RELA mRNA expression levels were the highest when treated with a mixture of the three cytokines. ** p < 0.01.