4. The investigator will select the appropriate treatment arm for the patient with the following requirements: (a) Patients cannot have had prior progression or intolerance to a specific Mab and then enrolled on an arm with that same Mab plus pembro, (b) The Mab on the arm selected must be considered standard of care or listed in the NCCN guidelines (www.nccn.org) for that cancer type. For Arm 1, patients with HER2 overexpressing MBC eligible for maintenance trastuzumab are allowed after taxane plus trastuzumab plus pertuzumab combination therapy.