Owing to the greater corneal neuropathy occurring in diabetic chronic kidney disease compared to type 2 diabetes alone [3], as well as the potential role of substance P in kidney function [11,12], we postulate that there may be differences in the tear film substance P concentration between individuals with type 2 diabetes with chronic kidney disease compared those with only type 2 diabetes. This evidence concerns the gene TAC1 and type 2 diabetes mellitus.