The interferon inducible p200 family proteins are expressed in a variety of cells and tissues. Intensive studies showed that they were involved in the regulation of cell proliferation and differentiation based on their ability to bind and modulate the activities of multiple transcription activators and inhibitors. Among p200 proteins, p203 has received the least attention and its function is unknown. In the present study, four multiple splicing isoforms of Ifi203, named temporally as Ifi203a/b-1, Ifi203a/b-2, Ifi203 a/b-3, and Ifi203a/b-4, were cloned from the interferon induced 10T1/2 cells. Anti-p203 antiserum was prepared and it could immunodetect a 46 kD and a 51 kD p203 proteins in a variety of cell lines. Unlike other p200 proteins, p203 was exclusively expressed in the liver in the adult C129/SvJ and C57BL/6 inbred strain mice. During the liver regeneration following a standard partial hepatectomy in C57BL/6 mice, the level of p203 decreased significantly in 6-24 h post-operation prior to the cell cycle progression through the G1/S transition.