Candidate biomarkers of 'physiological age' include a highly regulated set of energy metabolism genes. GO classifications and functional overrepresentation of aging biomarkers. Orange or blue text denotes up- or down-regulated genes, respectively. 'Count' refers to the number of genes in the gene set belonging to a particular GO category. 'Ref' refers to the number of genes belonging to a particular GO category represented in the reference list (DrosGenome1 array).