During 1976-80, V. Sasisekharan and his coworkers, of Indian Institute of Science, Bangalore, emphasized that the right handed form (R) of double helical B-DNA can not easily explain the unwinding sf the duplex due to topological difficulties. They suggested that right handed (R) and left handed (L) double helices are equally likely and therefore proposed a new RL model. In this model a B-DNA structure with alternating right handed and left handed segments of approximately five base pairs, in a repeat of ten base pairs, was proposed. This model provides for conformational flexibility. The essential differences between the right handed (R) double helix of Watson and Crick and the RL model of Sasisekharan are given in Table 25.3. There is strong experimental evidence available in favour of RL model, although one does not know the possible maximum continuous lengths of right handed and left handed segment of DNA.