A combination comprising two or more polynucleotides that are differentially expressed in fat animals compared to lean animals or two or more proteins produced by the expression of such polynucleotides is disclosed. The combination and probes based upon the combination are used for formulating a prognosis that an animal is likely to become fat, developing a diagnosis that an animal is fat, screening substances to determine if they are useful for modulating the amount of adipose tissue on an animal, and detecting the differential expression of one or more genes differentially expressed in fat animals compared to lean animals in a sample. Methods for using class predictor gene profiles to identify fat and lean animals are also disclosed.