This invention relates, e.g., to a composition that, at room temperature, when contacted with a sample comprising phosphoproteins, can fix and stabilize cellular phosphoproteins, preserve cellular morphology, and allow the sample to be frozen to generate a cryostat frozen section suitable for molecular analysis. The composition comprises (1) a fixative that stabilizes the proteins in the sample and that has a sufficient water content for a stabilizer and/or a permeability enhancing agent to be soluble therein; (2) a stabilizer, comprising (a) a kinase inhibitor and (b) a phosphatase inhibitor and, optionally, (c) a protease (e.g., proteinase) inhibitor; (3) a permeability enhancing agent; and (4) lactic acid. Methods and kits are described for preserving phosphoproteins, using such a composition. Also described are endogenous surrogate markers for monitoring protein degradation, including the loss of posttranslational modifications (such as phosphorylation), e.g. following removal of a cell or tissue from a subject; and exogenous molecular sentinels (e.g. phosphoproteins attached to magnetic nanoparticles) that allow one to evaluate the processing history of a cellular or tissue population sample.