Episode 28
Julia gives an explanation that makes like negative sense, but whatever, it's my canon:
JULIA: "As you all know, there are four DNA nucleotides: Adenine, Guanine, Thymine, and Cytosine. These are the common basic building blocks for all living things on Earth. Even the Chiropteran samples we have recently secured are no exception. However, when we examined the blood of Miyagusuku Riku, who has become Saya's Chevalier, we made a new discovery. It has become clear that his DNA contains a fifth nucleotide, which does not exist in any life form on this planet."
JOEL: "A fifth one?"
COLLINS: "And why is it that we haven't yet been able to detect this fifth nucleotide until now?"
JULIA: "It's because of the unique property of the fifth nucleotide...It can only be detected during a limited period of time. Once this nucleotide initiates gene expression, it will actively rewrite its own molecular structure."
JOEL: "Rewrite its own molecular structure?"
JULIA: "Then, with the formation of new DNA base pairs, a rapid transformation of the biological structures within the organism will follow. This is the secret of the transformation from human to Chiropteran. I have, for the time beind, named this nucleotide the 'D nucleotide,' D being the first letter in 'disguise.'" [REALLY JULIA? I am disappoint.]
COLLINS: "Disguise?"
JULIA: "Yes, Professor Collins. Once the D nucleotide expresses itself and changes its own structure, it will evade our detection. Furthermoe, we can explain the Chiroptera's superhuman abilities based on its properties. With the synthesis of new protein macromolecules, it is possible to have skin with the strength of steel and to achieve extraordinary muscle power. It is even possible for them to mimic other human beings and to possess other further-advanced abilities."
JOEL: "How should we fight against such an enemy? It's as though we're up against a completely superior being."
JULIA: "That isn't true, Director. If that were so, this planet would have been dominated by Chiroptera."
COLLINS: "However, in reality, that is not the case. There are things that we still need to learn."
JULIA: "That's right, Professor Collins."