Even though ISA Browns look like all the other red feathered-with-white-underfeathers red sexlinks, ISA Browns are a proprietary hybrid. They are a secret mix of several breeds, and can't be made by crossing two breeds like the other red sexlinks can. (Yes many hatcheries make their own red sexlink crosses and call them Golden Comets, Red Stars, etc.) Hatcheries buy ISA Brown eggs direct from the proprietary owner; they cannot create an ISA Brown through their own breeding.
While this may all sound "elitist" since in appearance ISA Browns resemble all the other red sexlinks, I've had ISA Browns, Golden Sexlinks, Red Stars, & red sexlinks resulting from a New Hampshire/Delaware cross. While all are great layers, eggs from ISA Browns are the most beautiful, rich golden brown of all, & egg size reaches jumbo within 6 months of POL. I sell eggs at a farmers market, so those are the reasons I only buy true ISA Browns as day old chicks now.