In contrast to the enzymes responsible for the earlier steps of the pathway that are encoded by single or small numbers of genes, the dioxygenases are encoded by multigene families, members of which differ in their positional and temporal patterns of expression. The phylogenetic relationship between the Arabidopsis GA20ox, GA3ox and GA2ox genes that have been identified on the basis of their derived amino acid sequences are shown in Fig. 10. Five GA20ox and four GA3ox genes have been identified. So far mutant phenotypes have been recognized only for AtGA20ox1 (ga5) and AtGA3ox1 (ga4), in both cases the mutants growing as semi-dwarfs (Fig. 3) with normal seed germination and flower fertility. There is, therefore, partial redundancy between the gene family members, which may have overlapping expression or, alternatively, GA products from other family