Most commonly accepted theory of the specificity is based on "Dscam" gene. "Dscam" gene also known as Down syndrome cell adhesive molecule is a gene that contains 3 variable Ig domains. Those domains can be alternatively spliced reaching high numbers of variations. It was shown that after exposure to different pathogens there are different splice forms of dscam produced. After the animals with different splice forms are exposed to the same pathogen only the individuals with the splice form specific for that pathogen survive.