Phase 1 Clinical Trial to Evaluate the Safety and Anti-Tumor Activity of AB-201

Inclusion Criteria: * ECOG performance status 0 to 1. * Histologically confirmed HER2 expressed breast or gastric/GEJ cancer IHC ≥ 2+ within 6 months prior to study entry. * Confirmed diagnosis of an advanced/unresectable or metastatic HER2+ breast or gastric/GEJ cancer that is refractory to, or intolerable of standard treatment, or for which no standard treatment is available. * Must have received prior cancer therapy: Subjects with breast cancer must have received ≥ 3 prior systemic therapies; subjects with gastric/GEJ cancer must have received ≥ 2 prior systemic therapy(ies); subjects with IHC 3+ or IHC 2+/ISH+ cancers must have received previous treatment with a HER2-targeting therapy. Exclusion Criteria: * Known past or current malignancy other than inclusion diagnosis. * Known clinically significant cardiac disease. * Active central nervous system (CNS) metastases, or involvement of the CNS, unless there is a history of at least 3 months of sustained remission. * Unresolved toxicities from prior anticancer therapy. * Ongoing uncontrolled systemic infections requiring antibiotic, anti-fungal, or anti-viral therapy. * History of sensitivity or intolerance to cyclophosphamide or fludarabine. * Currently Pregnant or lactating * Severe disease progression or health deterioration within 2 weeks prior to lymphodepletion regimen.