RNases H1 have been extensively studied to explore the relationships between structure and enzymatic activity. They are also used, especially the "E. coli" homolog, as model systems to study protein folding. Within the H1 group, a relationship has been identified between higher substrate-binding affinity and the presence of structural elements consisting of a helix and flexible loop providing a larger and more basic substrate-binding surface. The C-helix has a scattered taxonomic distribution; it is present in the "E. coli" and human RNase H1 homologs and absent in the HIV RNase H domain, but examples of retroviral domains with C-helices do exist.