Yet another suggestion that the line between pathological and functional mechanisms is not well defined:http://economist.com/science/displayStory.cfm?story_id=4078826"Brain development may be influenced by genetic parasites"
"[...]Nerve-cell precursors can turn into two types of brain cell besides nerve cells. These other two types have supporting, rather than starring roles in the brain, and cannot transmit nerve impulses. The rat-cell work showed that LINE-1 jumping happens only in precursors that turn into nerve cells, and that it seems to be regulated by a protein called Sox2 that is already known to play a crucial role in the formation of nerve cells. The mouse work showed that LINE-1 was not jumping in any other parts of the body (except, oddly, the sex cells—a result that had been seen before). That suggests it is happening in the brain for a purpose[...]"
(Also I've discovered that you can force LJ to e-mail your own comments to you, I have no idea, why I did not notice this option before; also I am probably going to Provincetown for the weekend --- no Internet access, I think.)