Further studies on r(CCCCGG)n(CCCC) (2 ≤ n ≤ 10) using CD and DSC confirmed the existence of noncanonical base pairs possibly caused by strand slippage, which was also observed in Xray constructs incorporating CCG repeats . Nevertheless, the fact that the crystal structure of a model r(CCCCGG)3(CCCC) was determined suggests that 2×2 CC/CC is less dynamic than 2×2 GG/GG . Having all these information available, we decided to study the 2×2 CC/CC loops using NMR spectroscopy and MD calculations. The antisense repeats, r(C4G2) exp , is predicted to form two types of 2×2 CC/CC internal loops; i) 5′-CCCG-3′/5′-CCCG-3′ (CCCG) and ii) 5′-GCCC-3′/ 5′-GCCC-3′ (GCCC). As a result, both of these systems were investigated to describe the structural details of 2×2 CC/CC internal loops formed by r(C4G2) exp . The same GNRA tetraloop described above, which was used to study 2×2 GG/GG internal loops, was utilized to probe 2×2 CC/CC internal loops (Figure ). The 5′-CCCG-3′/5′-CCCG-3′ sequence was flanked with neighboring AU base pairs to prevent strand slippage and eliminate spectral overlap to aid in NMR assignments (Figure ). In addition, a GNRA tetraloop cap was incorporated to enable distinct assignments for both the top and bottom helices (Figure ).