The centrosome. The centrosome consists of two centrioles that are formed from nine sets of microtubules (red tubes), which are triplet at the proximal ends and doublet at the distal ends of centrioles. The two centrioles attached to one another via their proximal ends by a flexible linker (green ribbons). Surrounding the proximal ends of each centriole is a matrix of proteins, the pericentriolar material (PCM) that is a site of microtubule nucleation as well as procentriole assembly (yellow ribbons). The two centrioles differ from one another, as one is slightly longer and also possesses two sets of appendages (distal and sub-distal drawn as orange sticks and red cones, respectively). This centriole is referred to as the mother while the other is the daughter centriole. PLK4 localizes to the proximal ends of both centrioles and the distal end of the mother centriole.