Methods, computer program products and systems for identifying cellular constituents in a secondary tissue that serve as surrogate markers for a target gene expressed in a primary tissue of a species are provided. A classifier is constructed using cellular constituent abundances of cellular constituents in a first plurality of cellular constituents measured in the secondary tissue in a population. This population comprises a first and second subgroup. The classifier is based on a second plurality of cellular constituents that comprises all or a portion of the first plurality of cellular constituents. Abundance levels of each cellular constituent in the second plurality of cellular constituents varies between the first and second subgroup. All or portion of the population is classified into a plurality of subtypes using the classifier. Then, one or more cellular constituents that can discriminate members of the population between a first subtype and a second subtype in the plurality of subtypes are identified.