(A) Abundance of PLK4, STIL or SAS-6 controls centriole numbers. Under physiological conditions, normal levels of PLK4, STIL and SAS-6 ensure the formation of only one procentriole per mother centriole (upper row). In contrast, depletion of either protein inhibits centriole formation (middle row) and, conversely, overexpression triggers the formation of several procentrioles per mother centriole (lower row). (B) Super-resolution immunofluorescence images illustrate the (near-) simultaneous formation of several procentrioles around each of the two mother centrioles of a U2OS cell conditionally overexpressing PLK4 [43]. Centrioles are stained for STIL (green) and CP110 (a marker for the distal ends; red). The right panel shows a ×10 magnification of the area marked in the left panel (DNA is shown in blue). Error bars denote 2 μM (left image) and 1 μM (right image). (C) Highly simplified scheme summarizing key interactions between components of the PLK4–STIL–SAS-6 module during cartwheel formation (for more detailed information see text and Figures 2 and 3).