CD99 and small cell carcinoma: Histological evaluation and the utilization of some IHC markers and antibodies, such as O13, HBA-71, and 12E7 (the MIC2 gene product) to recognize the cell surface antigen, defined by the cluster of CD99, facilitate the diagnosis.16 Small cell carcinomas exhibit consistent positive immunoreactivity to cytokeratins, and reactivity for chromogranin and thyroid transcription factor 1.