Abstract
The inner and outer dynein arms in eukaryotic flagella are key protein assemblies responsible for force production. Although the two types of arm have been believed to have similar properties, studies on newly isolated Chlamydomonas mutants lacking either of them indicate that they differ strikingly in structure and function; for example, flagella lacking the entire outer arms are motile, whereas those lacking the entire inner arm are nonmotile. These studies suggest that the inner and outer arms play different roles in flagellar motility, and raise a problem of how they cooperate to produce efficient flagellar beating.
