Background: PARP inhibitors exploit synthetic lethality in tumor cells with deficiency in homologous recombination repair (HRR). In line with this, most reported mechanisms of PARP inhibitor resistance restore HRR. Of multiple resistance mechanisms reported preclinically, reversion mutations in BRCA genes are the only confirmed mechanism of resistance to both platinum and PARP inhibitors in patients (pts) to date (Lin K et al. Cancer Discov 2019), with most studies focusing on ovarian cancer. MMEJ is an alternative DNA damage repair pathway, in which DNA polymerase θ (POLθ) has a key role; MMEJ has been suggested to play a role in BRCA reversion mutations (Tobalina L et al. Ann Oncol 2021). Methods: Targeted circulating tumor DNA (ctDNA) sequencing analyzed over 500 plasma samples collected at baseline and at progression to therapy in pts with ovarian or breast cancer and a mutation in BRCA1 and/or BRCA2 (BRCAm) who were treated with olaparib or chemotherapy in one of three Phase II/III clinical studies (LIGHT NCT02983799, SOLO3 NCT02282020, OlympiAD NCT02000622). Only pts with an original pathogenic BRCAm detected in ctDNA were evaluable. BRCA reversion mutations were identified using internal computational framework; DNA sequences surrounding BRCA reversion sites were analyzed for MMEJ signatures. Results: At baseline, in pooled data across treatment arms and across all available samples, BRCA reversion mutations were detected in 4/114 (3.5%) and 6/133 (4.5%) of breast and ovarian cancer pts, respectively, which may have developed on prior platinum therapy. At progression, BRCA reversion mutations were detected in 34/79 (43%) breast cancer pts and in 26/101 (26%) ovarian cancer pts who received olaparib, with at least 2/79 and 4/101 reversions already present at baseline, respectively. At progression, in the chemotherapy arm, BRCA reversion mutations were detected in 3/34 (9%) breast cancer pts and 1/29 (3%) ovarian cancer pts, with 2/34 and 0/29 reversions present at baseline, respectively. Reversion mutations varied in allelic frequency and were either present as single or multiple reversions, suggesting multiple events within the tumor were driving resistance. The location and type of reversion mutations reflected the functional importance of BRCA protein domains. A large proportion of BRCA reversion mutations (47/69 [68%] that were evaluable) were mediated by the MMEJ pathway based on the presence of MMEJ signatures around BRCA reversion sites. Conclusions: We detected BRCA reversion mutations in at least ̃40% of breast and ̃20% of ovarian cancer pts following treatment with olaparib. A large proportion of these reversion mutations are likely to have been mediated by MMEJ repair, suggesting that POLθ inhibitors in combination with platinum or PARP inhibitors might prevent or delay emergence of PARP inhibitor resistance. Clinical trial information: NCT02983799, NCT02282020, NCT02000622.