Expression distribution of mouse protein functional groups. (a) The metabolic functional group includes enzymes and transporter proteins. (b) The regulatory group includes signal transduction and transcription regulation proteins. The pre-metazoa group includes universal proteins and eukaryote specific proteins. The metazoan-specific group includes metazoan-specific and mammalian-specific proteins. The plot presents the fraction of proteins in a group that are expressed in N tissues. The analysis is restricted to those proteins expressed in at least a single tissue. Sample size (number of assigned proteins): pre-metazoa metabolic proteins 1370; metazoan metabolic proteins 267, pre-metazoa regulatory proteins 169; metazoan regulatory proteins 493. The error bars indicate the standard error estimate using bootstrap resampling.