With genome projects spewing forth DNA sequence at tens of Megabases per year, the problem of genetic nomenclature becomes daunting. In the field of cytochrome P450 (P450), there are more than 750 sequences and they are accumulating rapidly. At the current rate, there will be more than 1000 P450 sequences by late 1998 or early 1999. For these data to be accessible and useful they must be sorted and categorized in some meaningful way. To that end, a cytochrome P450 nomenclature system was devised (1). This system relied on evolutionary relationships as depicted in phylogenetic trees or dendrograms derived from the P450 protein sequences. The whole collection of sequences represents the P450 superfamily, with families and subfamilies being arbitrarily defined as distinct clusters on the tree. Orrgmally, 40% identity was used as a cutoff for family membership and 55% was used for subfamily membership. Since the inception of the system, both numbers have crept downward. The actual decision to mclude a sequence in an existing group largely depends on how it clusters on a tree and not so much on the absolute percent identity, which is more or less a rule of thumb. The most recent published compilation of the nomenclature is given in Nelson et al. (1996) (2). More up-to-date information is available at .