A gene probe for "rpoB" is available in some countries and this serves as a useful marker for MDR-TB, because isolated RMP resistance is rare (except when patients have a history of being treated with rifampicin alone). If the results of a gene probe ("rpoB") are known to be positive, then it is reasonable to omit RMP and to use SHEZ+MXF+cycloserine. The reason for maintaining the patient on INH despite the suspicion of MDR-TB is that INH is so potent in treating TB that it is foolish to omit it until there is microbiological proof that it is ineffective.