BK and explicit chiral symmetry breaking

The BK parameter is discussed in the general context of calculating beyond the factorization approximation for hadronic matrix elements. A variant of the 1Nc method of Bardeen et al. is used. We present calculations within a low energy approach and a calculation within the Nambu-Jona-Lasinio model. Matching with the QCD behaviour and dependence on non-zero current quark masses is studied.