In order to evaluate the minimal DNA binding length of TcaR, GC-rich fragments of different lengths were designed. As seen in Figure 1D , GC-rich fragments with 33, 29, and 25 bases showed similar binding strength to TcaR; with TcaR forming a large, apparently multimeric complex with GC33, a small complex with GC25, and both small and large complexes with GC29 in EMSA. These results indicated that the minimal observed ssDNA fragment size to allow TcaR binding ranges between 17 to 25mer; providing useful information for the design of a DNA fragment with precise length suitable for crystal packing. Up to now, only three MarR family protein complex structures have been reported, and the first one is complexed with dsDNA [13, 16, 22] . The second one is complexed with salicylate [12, 17, 22, 23] and we discovered the third case which is complexed with antibiotics [17] . We have already obtained TcaR-ssDNA crystals and collected X-ray diffraction data to 3.6 Å resolution at SPring-8 (Hyogo, Japan), beamline BL12B2. However, the phase problem is still the main challenge and the works are currently under progress.