The RFLP-PCR analysis of CYP2C19 variant alleles in whole blood is a widely used and well validated method . We have adapted this assay for determination of the CYP2C19 genotype from archival serum using published methods for DNA extraction from serum . Direct DNA sequencing of random samples was also undertaken, as a rare variant, which overlaps with the BamHI recognition sequence in exon 4, can result in a misclassification of the CYP2C19*3 variant when using RFLP analysis . There was agreement between the DNA sequence and the RFLP assignment for all the samples tested.
A high incidence (36 per cent) of the CYP2C19 genetic polymorphism has been reported previously in the East Sepik province of PNG . This is lower than the incidence predicted by Kaneko et al. (>74 per cent), however, based on the observed high frequency of CYP219 variants in Vanuatu [7, 19, 20].
Colonisation of the Pacific Islands is believed to have involved migration from South-east Asia, via PNG, through Melanesia and Micronesia into Polynesia [8–11]. The first migration wave is believed to have been from South-east Asia to PNG about 40,000 years ago, with a second wave of migration that extended into the Pacific (Remote Oceania) during the past 3,500 years. Thus, the high prevalence of variant CYP2C19 in Vanuatu may be due to genetic drift rather than a high incidence of the polymorphism in the migrating founder population. Alternatively, as PNG is a country of large ethnic diversity and comprises many geographically isolated populations, another PNG population may be the source of the high incidence of CYP2C19 variants in Vanuatu.
The present study has confirmed the high CYP2C19*2 and CYP2C19*3 allele frequency observed by Masta et al.  in an additional population that is geographically distant (Central province, south-east coast) from East Sepik (Northern Province). There is a significantly higher incidence of the CYP2C19*3 (c.636 G > A) variant allele in the Iruna population compared with the East Sepik population (0.34 vs 0.16) . The frequency of this variant allele was also originally reported to be low (0.133) in Vanuatu [7, 19]. A more detailed analysis of 24 island populations of Vanuatu , however, demonstrated considerable variability in the incidence of the CYP2C19*3 (c.636 G > A) variant allele (0.02-0.33). The *3 allele is absent in Caucasian populations  but is found in Asian populations at a frequency of 0.045-0.156 . To our knowledge, the highest incidence of the CYP219*3 allele reported is in Indonesian populations of south-east Asia (0.37); thus, the frequency of the *3 allelic variant in the Iruna population of PNG is one of the highest reported to date.
Although the incidence of homozygous *2 and *3 variants in this study was very high, no individuals homozygous for both allelic variants (ie c.681 A/A & c.636 A/A) were observed in any of the samples analysed. This supports the independent segregation of the two variant alleles, as reported previously . Moreover, the *2 allele frequency is consistently higher than the *3 allele in Asian populations , East Sepik , Vanuatu  and also the Iruna region of PNG in the present study. The combination of a high *2 and *3 allele frequency in the Iruna population of PNG, however, results in a high incidence of genotypic PMs (48.9 per cent). This PM incidence is intermediate between that reported for Indonesia (34 per cent ) and for Vanuatu (61.0-70.5 per cent [7, 20]) but higher than that reported previously for PNG (36 per cent ). 10.6 per cent of the Iruna population were homozygous *3/*3, whereas these individuals are absent or have a low incidence in East Sepik (0-2 per cent ) and Vanuatu (1.2 per cent; Figure 2). Throughout the Vanuatu archipelago, there is considerable variability in the incidence of *3/*3 individuals (0.0-16.0 per cent), with the majority of islands having a *3/*3 incidence of < 0.5 per cent . The incidence of the *2/*2 genotype is also highly variable within the archipelago (16-63 per cent). This large variability may be the consequence of the relative isolation of these island populations. Similarly, the high incidence of *3/*3 individuals in Iruna compared with the East Sepik population may also be the consequence of genetic drift of a relatively isolated population. Importantly, the differences in the incidence of not only the CYP2C19 PM genotype, but also the CYP2C19*3 allelic variant in the Iruna and East Sepik populations of PNG indicate that neither population is a likely 'source' of a founder population responsible for the high incidence of CYP2C19 variants in Vanuatu.
An increased incidence of genetic variants of another drug metabolising enzyme, CYP2B6, has also been reported in PNG. Compared with Caucasian populations, the incidence of the CYP2B6*6 variant is increased in PNG (0.28 versus 0.62, respectively), with a number of other, rarer variants observed at a low frequency, similar to that found in Caucasians . By contrast, the incidence of the CYP2C8*1 variant is reported to be low or non-existent in 285 individuals so far studied .
The results of the present study confirm the relatively high incidence of CYP2C19 variants in PNG reported previously . The relatively high CYP2C19 PM incidence in PNG could have profound consequences for the clearance of many drugs which are metabolised by this enzyme, including the anti-malarial drug proguanil [1, 24].
If the increased frequency of the CYP2C19*2 variant allele observed in Vanuatu occurred early in the colonisation of Remote Oceania, the other island populations of Polynesia and Micronesia may also have high PM frequencies. Relatively little is known about the CYP2C19 genetic status of Pacific Island populations. Phenotypic studies in New Zealand Maori of 'mixed racial background with Caucasian parentage'  and South Pacific Islanders who 'considered themselves to be > 75 per cent Polynesian' , however, indicate a PM incidence of 7 per cent and 13.6 per cent, respectively. Further studies to determine the incidence of the CYP2C19 PM genotype in these populations is warranted, as this enzyme is involved in the metabolism of an increasing list of drugs and the medical consequences in these populations may be important.