Rei's back was hurting him from the moment he was reanimated, 14 centuries after he was placed in cryo-hibernation. OMCOM had an opportunity to study his physiology and concluded that Rei's back pain was caused by a degeneration of his lumbar disks. Specifically, his annulus fibrosis was fissured and his interstitial fluid came in contact with the nucleus pulposus thus releasing Tumor Necrosis Factor Alpha, an extreme irritant. The mission planners did not realize that being frozen so long would cause this effect. The landing party of Ark I showed this effect as did Ark II and Ark III. We do not know much about the Ark IV or the Stealth Ark but we can assume they suffered from the same ill effects.
OMCOM desired to cure Rei's back pain but he could not help himself when it came to enhancing Rei's physiology. He started with the design of the 24th chromosome and made some changes. OMCOM had Rei take a yellow pill. Rome later took a similar one, after Aason was conceived but before Lupe was conceived.
To create his new chromosome, OMCOM took the DNA double-helix of MASAL's 24th chromosome and added an outer ring of protein, creating, in essence, a triple helix. You could call it PNA. Next, OMCOM took the genes contained within the chromosome normally responsible for the bloco (internal display tablet) and stilo (internal stylus) and modified them so that instead of being digital, they were analog. This produced the "cellphone" built into Rei's head which he used to communicate with MINIMCOM and later Rome via the E-M spectrum. Aason and Lupe were both born with this capability as well. When Rome was given this chromosomal adjunct, she acquired the "cellphone" but lost her bloco and stilo as they used the same portion of her brain.
OMCOM used the chromosome's capabilities to regenerate Rei's disks back to their original condition, better, in fact, to when they launched. You could say that he had the back of an 18-year old. The pill also strengthened Rei's core, kind of a turbo-charged instant Pilates which gave him great strength.
OMCOM's chromosome did not alter Rei's eyes to give him the telescopic vision of the Vuduri. Rather, it added an extra neural net connecting his auditory system to his visual cortex, giving him "sonar vision" which was very acute. Rei could "see" in the pitch black as long as there was some sound to "illuminate" the landscape. To allow Rei to sleep, OMCOM built in a level-detector so that when Rei's head was horizontal (like when he lay down), the subsystem cut out allowing him to sleep.
There was one other side effect that OMCOM forgot to mention until Rei was nearly 80 years old. The pill took a "snapshot" of his DNA on the day that he swallowed it. When Rei and Rome were old and fatigued, OMCOM used the yellow pill to restore their bodies to the same condition as they were when they were 25. Further, their aging process did not seem to kick in again. I am not sure how long they lived. I only know they outlived their children and grandchildren.