Here's what this number 16 looks like to someone with total colorblindness. It's much more common to have trouble telling apart shades of red and green, blue and yellow, or a mix of these.

"They can tell whether a light is on or off—if you shine a bright light at them, they might look ... optometrist's eye chart. After the trial gene therapy, four children with LCA-AIPL1 who were ...