Of the 123 different mutations, 92 were single base substitution. Of these, 67 resulted in single amino acid substitution (see Table 1), nine produced nonsense codons, resulting in premature termination of the protein transla­tion, and 16 resulted in splicing errors. The remaining 31 mutations  were 11 DNA insertions and 20 DNA dele­tions. Eighty-four cases were associated with clinical vari­ant 4, 25 with clinical variant 1, 10 with clinical variant 3, and four with clinical variant 2.

The two deletions that do not affect the HPRT protein size, and resulted in the clinical variants 1 and 2, are HPRT (Japan 3) and HPRT (Illinois), respectively (see Table 4). Hypoxanthine-guanine phosphoribosy transherase (Illinois) occurred at the extreme 5’ end of the coding region and the HPRT (Japan 3) occurred at the 3’ end. The remaining deletions resulted in clinical variant 4.

Only one insertion resulted in the clinical variant 3 (see Table 5). It is a 3 bp addition at nt 429 that resulted in the introduction of an additional amino acid without disturbing the normal reading frame [3]. The remainder produced clinical variant 4.

Four mutations that resulted in splicing errors pro­duced clinical variant 3. These are HPRT (JLY) [4], HPRT (LN11B) [5], HPRT (JC) [5] and HPRT (DB) [5], representing IVS-I,+1 (G®A), IVS-I,+1 (G®T), IVS-II,+1 (G®A) and IVS-V,+1 (G®A), respectively (see Table 2). The remainder produced clinical variant 4.

One nonsense mutation (HPRT GS) was reported as clinical variant 3 [6]. This is a C®T substitution in codon 170, resulting in Arg®Stop (see Table 3).