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T. M. Redmond is the first author of a 1998 study establishing an essential role for RPE65 in the mouse visual cycle. The team compared mice lacking RPE65 with control animals using electroretinography and biochemical measurements of vitamin A derivatives. The mutant mice lacked normal rod cell responses and rhodopsin, despite retaining the opsin component of the light-sensitive protein.^[ref]

This separates two possible explanations of failed light detection. A rod cell may contain opsin yet fail to assemble working rhodopsin if the required light-absorbing retinal is unavailable. The mutant mice accumulated one group of vitamin A derivatives while lacking the normal 11-cis products, linking the physiological defect to disrupted visual cycle chemistry. The experiment demonstrated a requirement for RPE65 in this pathway; deleting a gene alone does not identify every reaction performed by its protein.^[ref]

The visual cycle is the set of biochemical transformations that regenerates the light-sensitive retinal supply of visual pigments after illumination. In the classical rod pathway, a photon absorbed by rhodopsin converts its bound 11-cis-retinal toward all-trans-retinal. Subsequent processing involving the photoreceptor and the retinal pigment epithelium returns retinal to a form that can rebuild light-sensitive pigment. This recycling connects a rapid light-triggered molecular event to slower recovery of sensitivity.^[ref]

A useful experiment interrupted this supply rather than removing the receptor itself. T. M. Redmond and collaborators disrupted RPE65 in mice and measured retinal compounds and visual responses. The animals lacked normal rhodopsin and rod electroretinography responses even though rod opsin remained; all-trans retinyl esters accumulated, while 11-cis retinoids were absent. These results established that RPE65 is necessary for the normal production of 11-cis vitamin A derivatives in this pathway.^[ref]

The contrast supplies an illuminating example of biochemical dependence: a cell may retain a receptor protein yet lose the response because it cannot regenerate the small molecule required by that receptor. The visual cycle is the chemical recycling pathway; electroretinography measures an electrical consequence of its success or failure. The knockout establishes necessity in the tested pathway and conditions, rather than proving that every photoreceptor follows an identical regeneration route.