A new study found that a single gene, called callee oocyan, is responsible for the odd coloration of these blue chicken eggs. But how did it get there? Turns out that these chickens have a high incidence of a particular retrovirus, called EAV-HP. Retroviruses are a type of virus that integrates its own genetic data into the host in an unusual order. Instead of transcribing DNA into RNA and then into protein, retroviruses operate backwards, retroviruses have RNA, which they use to make DNA, and then integrate that DNA into the DNA of their hosts. HIV is probably the best-known retrovirus.