Acvr1CKO fiber cells that fail to withdraw completely from the cell cycle express fiber cell–specific markers Sections of P3 Acvr1WT and Acvr1CKO eyes were labeled with antibodies against markers characteristic of the terminally differentiated fibers c-Maf and MIP (Aquaporin0). Acvr1CKO fibers expressed c-Maf (B) and MIP (D), similar to Acvr1WT fibers (A, C). Some Acvr1CKO fiber cells expressed Ki67 and c-Maf (B, arrowhead, inset) or Ki67 and MIP (D, arrowhead, inset). LE, lens epithelium; LF, lens fiber cell; CE, ciliary epithelium. The white line separates LE from LF.