Background: The SMARCB1/A4 gene products are core subunits of the SWItch/Sucrose Non-fermentable (SWI/SNF) chromatin remodeling complex. Tumors with defects in SWI/SNF are histologically distinct aggressive cancers occurring in children and young adults. SMARCB1/A4 deficient tumors, particularly rhabdoid tumors, poorly differentiated chordoma, epithelioid sarcoma, and medullary renal cell carcinoma, have immune cell infiltrates and programmed death ligand 1 (PD-L1) expression. Response to immune checkpoint inhibition (CI) has been observed in SMARCB1/A4 deficient tumors; however, responses are not durable. T cell immunoreceptor with Ig and ITIM domains (TIGIT) is a novel inhibitory receptor expressed on multiple immune cells. TIGIT inhibits T and NK cells by binding to its ligand poliovirus receptor (PVR) and Nectin2 on tumor cells and antigen-presenting cells. Utilizing RNAseq data, SMARCB1/A4 deficient tumors demonstrate high expression of PVR and Nectin2. Tiragolumab is an antibody to the TIGIT receptor. In patients with non-small cell lung cancer, tiragolumab with atezolizumab increased survival compared to atezolizumab alone. Thus, data suggest that SMARCB1/A4 deficient tumors are susceptible to CI; however, monotherapy is unlikely to achieve durable responses; the addition of tiragolumab may enhance response rates. Methods: This is a phase 1/2 trial of tiragolumab monotherapy and in combination with atezolizumab in patients ≥ 12 months of age with SMARCB1/A4 deficient tumors administered IV on Day 1 of 21-day cycles. Part A will evaluate the safety of tiragolumab monotherapy (300 mg if ≤ 15 kg; 420 mg if >15 to ≤ 40 kg; 600 mg if > 40 kg or ≥ 18 yrs) based on cycle 1 DLTs in up to 6 evaluable patients <18 yrs of age. Part B will estimate the antitumor activity of tiragolumab in combination with atezolizumab (15 mg/kg [max 1200 mg]) if < 18 yrs or 1200 mg if ≥ 18 yrs) in 6 histology-specific cohorts (renal medullary carcinoma, malignant rhabdoid tumor, atypical teratoid rhabdoid tumor, poorly differentiated carcinoma, epithelioid sarcoma, and other) Each cohort will be conducted using a 6+4 Simon’s two stage design. Up to 13 patients may enroll in each cohort allowing for inevaluable patients (maximum n=78). Enrollment of patients ≥ 18 yrs on Part B may occur concurrently with enrollment of patients < 18 yrs on Part A. If the pediatric dose of tiragolumab is deemed safe, patients <18 yrs old may be enrolled on Part B. Cycle 1 toxicities of the combination therapy will be monitored in Part B using a Bayesian Optimal Interval Design in patients < 12 yrs of age. The secondary objectives are to characterize pharmacokinetics/anti-drug antibody and estimating progression free survival, overall survival, and duration of response. Enrollment is open for all Pediatric Early Phase Clinical Trial Network sites. Clinical trial information: NCT05286801.