A number of mutations in the X-chromosomal human iduronate-2-sulphatase gene have now been identified as the primary genetic defect leading to the clinical condition known as Hunter syndrome or mucopolysaccharidosis type II. The mutations that are tabulated include different deletions, splice-site and point mutations. From the group of 319 patients thus far studied by Southern analysis, 14 have a full deletion of the gene and 48 have a partial deletion or other gross rearrangements. All patients with full deletions or gross rearrangements have severe clinical presentations. Twenty-nine different "small" mutations have so far been characterised in a total of 32 patients. These include 4 nonsense and 13 missense mutations, 7 different small deletions from 1 to 3 bp, with most leading to a frameshift and premature chain termination, and 5 different splice-site mutations also leading to small insertions or deletions in the mRNA. A 60 bp deletion, that results from a new donor splice-site, has been observed in five unrelated patients with relatively mild clinical phenotypes. This information will not only be useful for MPS II patient and carrier diagnosis, but also will aid in the understanding of the structure and function of iduronate-2-sulphatase, and possibly in correlating genotype with phenotype.