Abstract Background: Improvement of systemic treatment of TNBC represents an unmet medical need. Recently, targeted therapy of regulatory immune pathways has become an important option in the treatment of numerous malignancies including breast cancer. Neoadjuvant trials combining chemotherapy and checkpoint inhibitors (KEYNOTE-522 and IMPASSION031) have demonstrated a meaningful benefit regarding pathological complete remission (pCR) and event-free survival (EFS) for the addition of PD-1- and PD-L1-inhibitors to chemotherapy among patients with TNBC, respectively. In addition, initial analyzes have presented promising results regarding event-free survival. In the neoadjuvant GeparNuevo trial only a subgroup of patients with TNBC, receiving a 2-week checkpoint inhibitor monotherapy window before the start of neoadjuvant chemotherapy in combination with checkpoint inhibition, achieved a pCR benefit as compared to patients treated with neoadjuvant chemotherapy alone. Trial Design: NeoMono is a phase 2 randomized multicenter trial recruiting male and female patients with primary TNBC (defined as ER/PR < 10% and HER2 negative). Neoadjuvant treatment in Arm A and B consists of Atezolizumab 1200 mg every 3 weeks in addition to neoadjuvant chemotherapy (i.e. 12 x Carboplatin and Paclitaxel q1w followed by Epirubicin and Cyclophosphamide q3w) In Arm A this therapy is preceded by Atezolizumab 840 mg q2w (Atezolizumab mono window). Study goals are the comparison of efficacy and safety of neoadjuvant chemotherapy with Atezolizumab with and without Atezolizumab two-week window (primary endpoint: pCR) and the identification of biomarkers predicting (early) response to or resistance against Atezolizumab. The broad translational program of the neoMono trial aims at identifying these biomarkers on tumor and patient level. The neoMono statistical design adapts the idea of a proof-of-concept trial and uses Bayesian posterior and predictive probabilities for inference about the primary hypothesis. Up to four planned efficacy interim analyses provide decision points for early stopping for success or futility. The expected maximum number of patients to be recruited is 458. Citation Format: Cornelia Kolberg-Liedtke, Johannes Schumacher, Ramona Erber, Michael Braun, Bernhard Heinrich, Oliver Hoffmann, Peter Fasching, Helmut Forstbauer, Anke Kleine-Tebbe, Georg Kunz, Michael Lux, Joachim Rom, Christian Schem, Nicole Stahl, Arndt Hartmann, Sabine Kasimir-Bauer, Hans-Christian Kolberg. neoMono - An adaptive randomized neoadjuvant two arm trial in patients with TNBC comparing a mono atezolizumab window followed by atezolizumab and chemotherapy with atezolizumab and chemotherapy [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT2-29-01.