Histopathologic examination of the nodule showed a diffuse infiltration of monomorphous mononuclear cells within the upper dermis. The cells contained centrally located oval nuclei. A Giemsa stain demonstrated metachromatic purple granules in the cytoplasm (Figure 4). Immunohistochemical studies showed that the mononuclear cells were positive for common leukocyte antigen and negative for S100 protein, HMB-45, and cytokeratins. The cells were therefore identified as mast cells.