Methods are provided for quantifying the cyclin-dependent kinase inhibitor 2A protein (p16) p16 protein directly in biological samples that have been fixed in formalin by SRM/MRM mass spectrometry. A protein sample is prepared from the biological sample using, for example, the Liquid Tissue reagents and protocol and the p16 protein is quantitated in the resulting sample by quantitating in the protein sample at least one fragment peptide from p16. Peptides can be quantitated in modified or unmodified form. An example of a modified form of a p16 peptide is phosphorylation of a tyrosine, threonine, serine, and/or other amino acid residues within the peptide sequence.
