ALK and lung cancer: In reference to lung cancer, where its importance has also been demonstrated both in the diagnosis and monitoring of these patients, authors such as Juan et al. or Pailler et al. have been able to detect ALK rearrangements in CTCs, which has allowed initiating therapy with tyrosine kinase inhibitors such as crizotinib, demonstrating its usefulness in the follow-up of patients diagnosed with lung cancer, although its sensitivity varies between 60–80%, which may limit its usefulness at the present time [42,43,44,45,46].