**3. The heads: Usefulness of IS***6110*

Soon after the discovery of IS*6110* as a specific element in MTBC, its usefulness as diagnostic tool was explored. Subsequently, at the beginning of the nineties it was demonstrated that two strains isolated of different episodes of a patient had the same IS*6110*-RFLP pattern, in turn, a high degree of polymorphism was observed between strains isolated from different patients (Otal et al., 1991). The fact that IS*6110* varies in copy number and location in the bacterial genomes, along with its stability over time showed their usefulness in genotyping of the MTBC. This IS has been successfully used throughout the world for identifying and characterize members of this complex.
