Suicide Squad: 30 Easter Eggs & References You Must See
11. Harley's Origin
Harley Quinn's origin mashes two important Quinn stories together. First, the actual story of her being a psychiatrist who develops a romantic obsession with The Joker comes from the classic Bruce Timm/Paul Dini story "Mad Love". Her doctor outfit is lifted out of that comic too.
And as the trailers confirmed a while ago, the actual process of her becoming Harley Quinn comes from the New 52 series, where the Joker also threw her into a vat of chemicals.
She even has the same outfit on.
The film very briefly flashes to the side of the chemical vat, which reads ACE. So that's why the Joker is drawn to it - it's where he was "born" too in The Killing Joke.