Fig. 1. Functional classification of 368 gene products that are most highly conserved in all three super-kingdoms (based on data obtained by whole proteome sequence comparisons of Homo sapiens, Drosophila melanogaster, Halobacterium halobium and Escherichia coli). Each protein was assigned one function when a major or well-understood function has been widely reported in the literature. Proteins with unknown or poorly characterized functions are contained in the set denoted `Other functions'. The number of proteins and % of total assigned to each function are listed. Basic biological functions that are closely tied to fundamental aspects of the cellular stress response are underlined. The extraordinary conservation of such functions is a strong argument for the monophyletic origin of a core stress proteome at an early stage of cellular evolution.