also been reported, for example on the C-helix, SH2 domain, substrate binding site, activation loop and C-terminal lobe. Some of these mutations have clinical significance, but none as much as P-loop and T315I mutations. T315I mutation The T315I is a unique mutation because of its resistance to all approved Bcr-Abl inhibitors, prior to ponatinib. It is caused by a single cytosine to thymine (C -> T) base pair substitution at position 944 of the Abl gene (codon '315' of the Abl protein) sequence resulting in amino acid Threonine being substituted by Isoleucine at that position - thus 'T315I'. This substitution eliminates a critical oxygen molecule needed for hydrogen bonding between imatinib and the Abl kinase, and also creates steric hindrance to the binding of most TKIs. When discovered, it was estimated that every 6 out of 9 cases of advanced stage CML with imatinib resistance carried this mutation. T315I produces the highest magnitude of resistance of any mutation both to imatinib and second generations TKIs. Ponatinib (Iclusig) by Ariad was approved in 2013 for use as second-line CML treatment, and is the only licensed TKI which binds to the T315I mutated kinase successfully. P-loop mutations The structure of Bcr-Abl contains two flexible loops, the ATP-binding P-loop and the activation loop. These loops have specific arrangements in the inactive conformation of Bcr-Abl that stabilize the basal conformation. Mutations in these loops destabilize arrangement of the loops such that the kinase domain cannot assume the inactive conformation required for imatinib binding. Mutations in the P-loop region are the most common, accounting for 36-48% of all mutations. There are clinical data indicating that Bcr-Abl mutations in the P-loop is 70-100 fold less sensitive to imatinib compared with native Bcr-Abl. Bcr-Abl Independent mechanisms of resistance Additional mechanisms have been postulated to describe resistance seen in various model systems although none have been clearly identified as a sole source of clinical resistance. Drug efflux caused by P-glycoproteins Some investigations in cell lines have shown that imatinib resistance may be partly due to an increase in the expression of the P-glycoprotein efflux pump. By utilizing agents that inhibit P-glycoprotein activity imatinib susceptibility has been restored in some cases. Drug import by organic cation transporter 1 The entry of imatinib into cells is dependent on an organic cation transporter (OCT1). OCT1 plays a significant role in imatinib resistance by inhibiting its influx and thus decreasing the intracellular