I'm doing some research in advance; in a future entry in my series, I plan to reveal one of the villains is a genetically engineered "person" - his DNA is comprised of 8 different nucleotides; 4 naturally occurring, 4 synthetic, but widely used in single cell experiments (in the story). This renders him effectively immune to all naturally occurring viral infections. While he was created to look human on the outside, he has various changes to his internal organs, including; A gland for producing ATP, along with a bladder which stores it. This allows the rest of his cells to operate without oxygen, as long as his ATP bladder isn't empty. The gland is the only organ which requires oxygen in large amounts. Two circulatory systems: One for oxygenated blood from the lungs to his ATP gland, another which carries only ATP and blood plasma around the rest of his body. This requires him to have 2 hearts. Would it be possible for him to experience muscle cramps? I understand these are caused by a build up of lactic acid, but without his muscles directly burning oxygen to generate ATP (as the chemical is fed to them by his wider circulatory system), would they generate any lactic acid at all? Also, since his genetic code has been built from the ground up, he technically isn't human, even though he creates all of the same proteins and most of the same organs. If you cut his arm (for example), he doesn't bleed red blood, but pale yellow blood plasma; only a deep wound to his ATP system would cause him to bleed red blood, like a normal person. I'll take suggestions for what to call his species (he's the only survivor of his creation experiment out of 6 individuals), and the supplier of the best suggestion will have a character named after them in the series, if they wish.