A schematic model of CP110 function in Drosophila. (A) In WT cells CP110 (green) performs two important functions: (1) it restricts the ability of core duplication proteins such as DSas-6, Ana2/STIL, and DSas-4/CPAP (shown collectively in yellow) to promote centriole elongation. This function can be performed by either CP110L or CP110S. (2) It prevents the MTs (purple) from extending beyond the distal end of the centriole. This function can only be performed efficiently by CP110L. (B) In the absence of CP110, the core centriole structure is slightly elongated, and the centriolar MTs can dramatically extend beyond the distal end of the centriole. (C) If certain centriole duplication proteins are overexpressed, a third function of CP110 is revealed (3), as it helps to suppress centriole overduplication. (D) If these duplication proteins are overexpressed in the absence of CP110, centriole overduplication is strongly enhanced. The mechanism by which CP110 suppresses this overduplication is unknown, but one possibility is depicted here, where the presence of elongated centrioles itself promotes overduplication by providing a larger platform for daughter centriole assembly.