RSPO1 and pancreatic insulinoma: We subsequently sought to identify the cell type specifically expressing Rspo1. Toward this goal, we analyzed the mRNA levels of Rspo1 in four different pancreatic cell lines: the 266-6 acinar cell line, the alpha-tumor cell line α-TC1, the mouse insulinoma β cell line MIN6, and the rat insulinoma β cell line INS-1 (Figure 1A).