The tree (grey dots and connecting paths) represents those categories (dots) in the gene ontology hierarchy for biological process that matched one or more genes expressed in liver or heart. Superimposed on the tree are the biological processes that matched one or more genes in the liver circadian set (green dots), the heart circadian set (red dots), or both (yellow dots). Marked are atypical examples of process clusters matching only to the circadian gene set from liver (green arrow) or heart (red arrow). Selected examples of biological process categories are indicated by the following numbers: 1, developmental process; 2, death; 3, cell growth and/or maintenance; 4, cell communication; 5, behaviour; 6, transport; 7, stress response; 8, metabolism; 9, (metabolism) nucleobase/nucleoside/nucleotide; 10, (metabolism) amino-acid derivative. More than 1,400 biological process categories are shown. Storch et al, nature 2002