The current disclosure provides specific peptides, and derived ionization characteristics of the peptides from the estrogen receptor (ER), progesterone receptor (PR), and/or antigen Ki67 (Ki67) proteins that are particularly advantageous for quantifying the ER, PR, and/or Ki67 proteins directly in biological samples that have been fixed in formalin by the method of Selected Reaction Monitoring/Multiple Reaction Monitoring (SRM/MRM) mass spectrometry. Such biological samples are chemically preserved and fixed wherein the 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 a biological sample using the Liquid Tissue™ reagents and protocol, and the ER, PR, and/or Ki67 proteins are quantitated in the Liquid Tissue™ sample by the method of SRM/MRM mass spectrometry by quantitating in the protein sample at least one or more of the peptides described for one or more of the ER, PR, and/or Ki67 proteins. These peptides can be quantitated if they reside in a modified or in an unmodified form. An example of a modified form of an ER, PR, and/or Ki67 peptide is phosphorylation of a tyrosine, threonine, serine, and/or other amino acid residues within the peptide sequence.