Phylogenetic tree of homologs of three human hyaluronan synthase (HAS) proteins that were proposed as lateral transfers from bacteria to vertebrates (3). Homologs of the human HAS genes were identified using iterative Blastp searches of a low redundancy protein database and aligned using clustalW. More distantly related proteins were used as outgroups to root the tree. The tree was generated from the alignment (variable regions and gaps excluded) using the neighbor-joining algorithm implemented by Phylip (25) using a PAM-based distance matrix. Species names, major evolutionary groupings, gene names if available, and sequence IDs (gi for Genpept and sp for Swissprot), are indicated in the tree. Scale bar corresponds to estimated evoluti nary distance units. The presence of multiple HAS genes in different vertebrate species is likely due to duplication in vertebrates.