The skin biopsies had similar histology to the synovial biopsies, in addition to absence of Birbeck granules on electron microscopy, positive staining for vimentin and lysozyme, prominent elastophagocytosis, and negative staining for Factor XIII-a, CD68 (KP-1), CD34, procollagen I, pan-actin, P75, CD45-RO, CD45-RA (Cox et al. 2001). The gene discussed is PTPRC; the disease is rheumatoid arthritis.