Two recent papers appearing in Nature describe the crystal structures of the motor domains of two members of the kinesin superfamily . A surprising finding was that not only do the two kinesin family members have highly homologous structures, but they also share many structural features with the motor domain of myosin! The studies represent a collaboration between the laboratories of Robert Fletterick, the crystallographer, and Ron Vale, who, along with Michael Sheetz and Tom Reese, first purified and named kinesin in 1985 . In addition, they were fortunate to have the collaboration of Roger Cooke, who brings a wealth of understanding of chemico-mechanical processes. Kinesin and NCD are from distinct subclasses of the kinesin superfamily . Kinesin, the founding member of this superfamily, has been widely studied and characterized in recent years (see Bloom & Endow, 1995, for review). Its motor domain is composed of approximately 340 amino acids located at the amino terminal end of a 90 000-118 000 dalton heavy chain. Much of the rest of the heavy chain forms a coiled-coil homodimer which also associates with two more globular light chain subunits that bind to the carboxyl end of the elongated molecule. Kinesin moves toward the plus end of a microtubule in steps of 8 nm in a highly processive manner (Svoboda et aI., 1993). This later phenomenon is clearly illustrated from observations that a single kinesin bound to a glass surface can move a microtubule . Recent studies of the microtubule-activated MgATPase activity of kinesin provide a kinetic basis for this processivity . The other structure determined is of the motor domain of NCD, which was discovered in Drosophila from molecular analysis of a chromosomal segregation mutant . While NCD contains a similar motor domain to kinesin by sequence alignment of the amino acids, its motor domain is located in the carboxyl terminal portion of the heavy chain 0142-4319/96