User blog comment:Oodronesoo/Crazy Theory/@comment-179280-20131111173554/@comment-12642484-20131126210503

Maybe Eren has been a Titan Shifter since birth and the injection from his father was just the coordinate. Or it is as you say and the Titan Shifting is a(n obviously lucky) side effect for Eren. Or maybe he had both of them since birth and the injection was something even completely different from those 2 abilities.