In the future, nothing is sacred, not even DNA. As one Dr. Abel Cuvier proved when he cracked the genetic code to allow animal DNA to be spliced safely into our human ones, make perfect hybrids not so affectionately called "Splicers". Skin, eyes, limbs, almost anything he could replicate, even to an insane degree.
Naturally, this took a darker turn when people began splicing to commit crimes, when Cuvier was discovered to be behind this, Batman investigated, which led to Cuvier injected himself with his serums, becoming a literal Chimera of almost unstoppable might, until Batman turned the tables and stopped him altogether.
Despite this, splicing is still a big part of the culture, and Batman deals with them often.