Pso p27 is a protein antigen expressed in psoriatic lesions and is shown to participate in complement activating immune complexes in the affected skin. The objective of the present study was sequencing of the Pso p27 protein in an approach to identify a "Pso p27 gene". The analyses showed that Pso p27 represents several proteins with homologies to various Squamous Cell Carcinoma Antigens (SCCAs). The unique N-terminal and C-terminal ends of Pso p27, however, differ from the terminal ends of the SCCA molecules and indicate posttranslational digestions of SCCA molecules with highly specific endoproteases. These structural variations may play crucial roles with respect to the immunogenicity of Pso p27 and the failure of immunological tolerance.