ABSTRACT The current disclosure provides for specific peptides, and derived ionization characteristics of the peptides, from the Insulin-Like Growvth Factor 1 Receptor (IGF- I R) protein that are particularly advantageous for quantifying the IGF-IR protein directly in biological samples that have been fixed in formalin by the method of Selected Reaction Monitoring (SRM) mass spectrometry, or what can also be termed as Multiple Reaction Monitoring (MRM) mass spectrometry. Such biological samples are chemically preserved and fixed wherein said biological sample is selected from tissues and cells treated with formaldehyde containing agents/fixatives including formalin-fixed tissue/cells, formalin fixed/paraffin embedded (FFPE) tissue/cells, FFPE tissue blocks and cells from those blocks, and tissue culture cells that have been formalin fixed and or paraffin embedded. A protein sample is prepared from said biological sample using the Liquid TissueTM reagents and protocol and the IGF- I R protein is quantitated in the Liquid TissueTM sample by the method of SRM/MRM mass spectrometry by quantitating in the protein sample at least one or more of the peptides described. These peptides can be quantitated if they reside in a modified or an unmodified form. An example of a modified form of an IGF-I R peptide is phosphorylation of a tyrosine, threonine, serine, and/or other amino acid residues within the peptide sequence.