Billy Prion's streak is by reason of his having deliberately had the left side of his mouth paralyzed with Botox for the first BSE gigs, and because, when Margot was taking her NYU extension course in disease-asmetaphor, Cayce had suggested she do something with his mouth. Margot, struggling to outline a paper in which Bigend was the disease she needed to find a metaphor for, hadn't been interested.