Illustration of an axoneme, an assembly of microtubules. Microtubules form part of the cytoskeleton inside cilia and flagella called the axoneme. The axoneme of a motile cilium has two central microtubules and nine outer doublets (called a 9+2 axoneme). The axonemal cytoskeleton acts as a scaffolding for various protein complexes and provides binding sites for molecular motor proteins such as kinesin II, that help carry proteins up and down the microtubules.