Microtubules (MTs) are hollow core cylinders that measure 25 nm in diameter. The walls are primarily composed of dimers of the proteins α- and β-tubulin (see Dustin 1984 for review). The dimers are arranged in 13 strands or protofilaments that are aligned almost parallel to the longitudinal axis of the MT (Tilney et al. 1973; Crepeau et al. 1978). The individual monomers in adjacent protofilaments are not aligned in register along the perpendicular axis of the tubule, but are staggered at an oblique angle and follow a helical path around the tubule (Amos 1979). This monomer helix has been shown to be a 3-start helical family with a pitch of 12 nm and a pitch angle of 10.5° for 13 protofilament MTs.