The Z-DNA domain of human ADAR1 (Zα ADAR1 ) produces B-Z junction DNA through preferential binding to the CG-repeat segment and destabilizing the neighboring AT-rich region. 9 However, this study could not answer the question of how many base-pairs in AT-rich region are destabilized by binding of Zα ADAR1 . Thus, we have performed NMR experiments of Zα ADAR1 to the longer DNA duplex containing an 8-base-paired (8-bp) CG-repeat segment and a 12-bp AT-rich region. This study revealed that Zα ADAR1 preferentially binds to the CG-repeat segment rather than AT-rich region in a long DNA and then destabilizes at least 6 base-pairs in the neighboring AT-rich region for efficient B-Z transition of the CG-repeat segment.