['Quantitative phosphoproteomics represents a front line for functional proteomics and hence for systems biology. Here we present a new application of the surface-activated chemical ionization (SACI) technology for quantitative phosphoproteomics analysis. The main advantages of SACI-MS technology are high sensitivity, quantitative accuracy and matrix effect reduction, which allow quantitative estimations.', 'A SACI-MS approach was used to investigate the quantitative in vivo phosphorylation of the cyclin-dependent kinase inhibitor Sic1, a low-abundance protein of Saccharomyces cerevisiae, which is phosphorylated on Ser201 by casein kinase 2 (CK2) and compared its phosphorylation status in cells growing in two different carbon sources (glucose or ethanol).', 'Our relative quantification indicated that the Sic1-Ser201 phosphorylation level is about 2-fold higher in ethanol- than in glucose-growing cells, proportional to the Sic1 protein level. This finding is coherent with results of western blot analysis using anti-phospho-Ser201-specific antibody, validating the results obtained with this new SACI approach.'] Does a surface-activated chemical ionization approach allow quantitative phosphorylation analysis of the cyclin-dependent kinase inhibitor Sic1 phosphorylated on Ser201?