An NsiI polymorphic site has been found in the human long QT intronic transcript 1 (LIT1). In this transcript, we found a C-to-T transition, which was located between exons 10 and 11 of KVLQT1, and was confirmed by sequencing analysis. The allelic frequency of this polymorphism, was 0.82: 0.18 in Japanese individuals. Our novel polymorphism, combined with other polymorphisms, could be very useful in helping to determine whether the imprinting of LIT1 is disrupted in Beckwith-Wiedemann syndrome (BWS) or in human cancers.