We consider a special class of dimension-six operators which are assumed to be induced by some new physics with typical scales of order Lambda. This special class are the operators with two quarks which can mediate transitions between quark flavours. We show that under quite general assumptions the effect of these operators can be parametrised in terms of six parameters, leading to a modification of the (at tree level flavour diagonal) neutral currents and to an ``effective CKM matrix for the charged currents, which is not necessarily unitary any more. The effects of these operators on charged and neutral currents are studied.
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