Pedi-STAR: Surface Target Antibody-drug Conjugate (ADC) Antigen Expression in Relapsed/Refractory Pediatric Solid and CNS Tumors

This study is looking to learn whether a tumor testing approach can feasibly measure antibody-drug conjugate (ADC) target proteins in children and young adults with relapsed or refractory solid tumors or central nervous system (CNS) tumors. The tumor testing approach is a two-step tumor profiling process of: * Screening with bulk RNA sequencing to see which ADC target genes are turned on in the tumor. * Confirmatory immunohistochemistry (IHC) for targets that show positive expression on the RNA screen, to confirm protein expression.