Two microlitres proteinase K (600 ¹g/ml) (Promega Benelux, Leiden, The Netherlands) were added and the tubes were incubated at 65±C (1 h) and 95±C (10 min) consecutively and finally centrifuged (1 min; 16 000 g). The DNA suspension was stored at ¡20±C and used for further study.