Axonemal dynein molecules are organized with heavy chains that form heterotrimers, heterodimers, or monomers, together with intermediate, light intermediate, and light chains. The number of heavy chains in outerarm dyneins depends on the species of origin: Outer-arm dyneins from most sources consist of two distinct heavy chains[20, 42], whereas those from Tetrahymena and Chlamydomonas each contain three distinct heavy chains. Inner-arm dyneins contain one or two heavy chains[16, 60-62] and at least seven subspecies identified in Chlamydomonas axonemes[35].Each dynein heavy chain consists of a C-terminal toroidal-shaped head domain ~13 nm in diameter, together with two elongated flexible structures called the stalk (~15-nm-long coiled coil) and the N-terminal tail (the cargobinding domain, formerly known as the stem), to which the intermediate chains (40-120 kDa) and most light chains (8-30 kDa) bind (Fig. 9.1). The toroidal-shaped head contains ATPase sites and the stalk carries at its distal end the ATP-sensitive microtubule binding site within a small ~4-nm-diameter domain. The N-terminal tail is thought to be involved in binding the dynein to cargoes (for cytoplasmic dyneins) or A-microtubule of the doublet in an ATP-insensitive manner (for axonemal dyneins).