MUC5AC and dry eye syndrome: [20] In some forms of dry eye, tear film instability may be the initiating event, unrelated to prior tear hyperosmolarity. [23] Thus, tear film instability can be caused by epithelial damage involving cell death by apoptosis, a loss of goblet cells, and disturbance of mucin expression. [23] Moreover, in all forms of dry eye, tear film instability exacerbates ocular surface hyperosmolarity and promotes the vicious circle of dry eye syndrome. [24]