Sulfotransferase (ST) enzymes catalyze the sulfate conjugation of many hormones, neurotransmitters, drugs, and xenobiotic compounds. These reactions result in enhanced renal excretion of the sulfate-conjugated reaction products, but they can also lead to the formation of "bioactivated" metabolites. ST enzymes are members of an emerging gene superfamily that presently includes phenol ST (PST), hydroxysteroid ST (HSST), and, in plants, flavonol ST (FST) "families," members of which share at least 45% amino acid sequence identity. These families can be further subdivided into "subfamilies" that are at least 60% identical in amino acid sequence. For example, the PST family includes both PST and estrogen ST (EST) subfamilies. Amino acid sequence motifs exist within ST enzymes that are conserved throughout phylogeny. These signature sequences may be involved in the binding of 3'-phosphoadenosine-5 '-phosphosulfate, the cosubstrate for the sulfonation reaction. There are presently five known human cytosolic ST enzymes: an EST, an HSST, and three PSTs. cDNAs and genes for all of these enzymes have been cloned, and chromosomal localizations have been reported for all five genes. Genes for these human enzymes, as well as those of other mammalian cytosolic ST enzymes that have been cloned, show a high degree of structural homology, with conservation of the locations of most intron/exon splice junctions. Human ST enzyme expression varies among individuals. Functionally significant genetic polymorphisms for ST enzymes in humans have been reported, and other molecular genetic mechanisms that might be involved in the regulation of the expression of these enzymes are being explored. Knowledge of the molecular biology of cytosolic ST enzymes, when placed within a context provided by decades of biochemical research, promises to significantly enhance our understanding of the regulation of the sulfate conjugation of hormones, neurotransmitters, and drugs.