Inclusion Criteria:  Patients with histologically documented diagnosis of Mantle Cell Lymphoma (MCL) as defined in the 2016 edition of the World Health Organization (WHO) classification, with available tissue for revision and additional studies; Diagnosis of Mantle Cell Lymphoma (MCL) between 1st of January 2008 and 30th of June 2020; Adults, 18-80 years at diagnosis; Relapsed or refractory disease after rituximab and chemotherapy containing induction regimens with curative intent. Treatment at relapse or progression on an intention-to-treat basis (ITT): at least one cycle of Chemo-immunotherapy (CIT), Bruton Tyrosine kinase inhibitors (BTKi), or alternative drugs combination; Subject understanding and voluntarily signing an informed consent form approved by an Independent Ethics Committee (IEC), prior to the initiation of any study-specific procedures.  Exclusion Criteria:  Unavailability of the samples requested by the study; Any histology other than Mantle Cell Lymphoma (MCL); Patients treated with front line regimens containing only rituximab or with palliative therapy; Untreated patients; patients undergoing watchful waiting approach.