Phosphorylated CBL does not ubiquitinate EGFR kinase domain mutants efficiently, which enables mutant proteins to escape degradation. There are indications that phosphorylated CBL shows decreased affinity for EGFR kinase domain mutants compared to wild-type EGFR proteins, and quickly dissociates, before ubiquitination is completed. This decreased affinity may be due to altered structure of EGFR kinase domain mutants or to the presence of the chaperone protein HSP90 in complex with the mutant protein. Weaker afinity for phosphorylated CBL was directly demonstrated for EGFR L858R mutant (Yang et al. 2006), and poor ubiquitination inspite of CBL binding was shown for EGFR L858R and EGFR E746_A750del mutants (Yang et al. 2006, Padron et al. 2007).