Phosphorylation levels of S519
in vivo by mass spectrometry analysis. Cycling and G1 cells obtained with α factor (red circles) or by G1‐cyclin depletion and
CLN3 induction (orange circles) were used to determine relative S519 phosphorylation levels as described under
Materials and Methods, and obtained ratios for three independent experiments are plotted. Error bars correspond to the standard deviation estimated from measurements of control peptide ratios in individual samples (
N = 3). The null hypothesis of a true ratio equal to 1 was tested and the resulting
P‐values are indicated assuming that control and problem peptide ratios are homoscedastic and normally distributed.