The Hedgemann's Serum variant they'd injected into Frank was about 70% of raw serum itself; a particular formulation that had seen some success in producing superhuman strength and durability. About 20% was a personality agent to make the physical changes less jarring and allow them to be integrated into the subject's psyche faster. The final 10% was a fairly untested compound they believed enhanced the effects of Hedgemann's Serum.
The enhancement agent worked sadly too well; enhancing both the superpowers being integrated into Frank's DNA, and the side effects. Frank groaned as he felt himself shrinking down, and down, his limbs retracting a little until he was a petite 5'2'', looking down in dismay at his weedy delicate arms. "Hey! Isn't this meant to make me superhuman?" he complained, realising his voice sounded more high-pitched, more perky, more... feminine! "Get me a mirror!" he demanded.
A technician rushed over, showing Frank his own face as it became soft, feminine, youthful. He looked like an 18 year old girl, dirty blonde hair cascading to his shoulder in waves as big brown eyes opened wide in dismay. The hospital gown was loose on him how, as he ripped it off to look at his changing figure. His waist narrowed and narrowed as his tummy drew taut; his hips widening, ass becoming round and pert, a backside you could bounce a quarter off of.
Frank squealed. "What the fuck, guys?!" He staggered back, throwing out one slender hand to steady himself on the hospital bench he'd been sat on. The legs buckled instantly, the steel folding like cheap paper as Frank let out an even higher pitched moan, his cock vanishing as a cute little snatch appeared on his body, just as a pair of D cup breasts, almost oversized on his tiny frame, exploded forth from his chest.
Humiliated, he was bundled under a blanket and taken to be poked and prodded by a seemingly endless parade of machines and technicians and scientists, his brain reeling from the realisation he was now... well, this!
And so he found himself back in the room where he started, in front of one of the older scientists. "Well, son, I have mixed news for you. The bad news is, the Serum reacted with you in such a way that you're now biologically a 16 year old girl."
Frank seethed, folding his arms. "Well, no shit."
The scientist raised a hand placatingly. "Well, the good news is you seem to be superhumanly strong and tough. We can figure out just how strong and tough, but well... you can be a superhero!"
Frank's jaw dropped. "A superhero? Aren't we gonna deal with me being a chick? What now?"