Michael Sheetz and James Spudich developed the first biochemical assay to examine the movement of myosin along actin filaments. Using myosin bound to fluorescently labeled beads, they tracked myosin movement microscopically along actin-aligned filaments, first in the alga Nitella (6) and later using purified actin bound to glass slides (7). Spudich and colleagues later showed that the S1 head of myosin both binds to actin and hydrolyzes ATP to generate motion (14).