DNA was extracted using the QIA amp DNA stool mini kit
(Qiagen, GmbH, Hilden, Germany). To minimise the risk of contamination,
a moderate number of target samples (8–12) were handled
simultaneously during DNA extraction, and always together with
a negative extraction control. Moreover, special facilities for lowcopy
number DNA research (separate bench, private pipettes, filter
tips) was used during extractions and PCR set-up.