When describing mutations in "rpoB" in other species, the corresponding amino acid number in "E. coli" is usually used. In "Mycobacterium tuberculosis", the majority of mutations leading to rifampicin resistance are in cluster I, in a 81bp hotspot core region called RRDR for "rifampcin resistance determining region". A change in amino acid 531 from serine to leucine arising from a change in the DNA sequence of TCG to TTG is the most common mutation. Tuberculosis resistance has also occurred due to mutations in the N-terminal region of "rpoB" and cluster III.