(a) The CETN1-GFP signal is brighter in the upper centrosome/spindle pole of a mitotic spindle. The merged differential interference contrast microscopy image with CETN1-GFP labelling at two centrosomes shows a metaphase chromosome. Insets: enlargements and semi-quantitative integrated intensity profiles of centrioles. Scale bar, 5 μm. (b) The brighter CETN1-GFP signal represents the older centrosome, as it co-stains more intensely for hCenexin1 and remains more intense throughout cell division (Supplementary Fig. S1a). Scale bar, 5 μm. Lower left, merge. 