NPTN and in situ carcinoma: Contraction of φCD508 in urea reduces the tail length by just 20%, whereas most CIS tails that have been described contract to ∼50% of their extended length; the outer diameter of the sheath increases from 230 to 250 Å, and the inner diameter from 78 to 94 Å, sufficient to break contacts between gp55 sheath domain I helices and the tail tube gp56 (Fig 3D).