Background: According to advanced neoadjuvant chemotherapy (NACT), which dual HER2 blockage in HER2+ BC and adding carboplatin or immunotherapy in triple-negative breast cancer (TNBC) increased pathologic complete response (ypCR) rates up to 68% and 80%. Because ypCR patients showed better survival, the indications for NACT have expanded to early BC and the expected ypCR rate has increased. Therefore, it may be reasonable to consider omitting surgery in cases with excellent responses to NACT. Several recent retrospective studies and pilot prospective studies have reported axillary pathologic complete response (ApCR) is highly correlated with breast pathologic complete response (BpCR) in HER2+ or TNBC after NACT. The current report describes the design of omission of sentinel lymph node biopsy (SLNB) after NACT in HER2+ or TNBC patients with excellent response. Similarly, EUBREAST-01 (NCT04101851) study is ongoing omission of sentinel lymph node biopsy (SLNB) after NACT in HER2+/TNBC with radiologic complete remission and a BpCR. Methods: The ASLAN trial is a prospective, multicenter, single-arm, clinical study. Five tertiary care hospitals in South Korea are participating. Inclusion criteria: women aged 20-69; cT1-3N0-1M0; HER2+ or TNBC (defined by ER-negative (< 10% positive cells in IHC) and PgR-negative (< 10% positive cells in IHC)); expected complete remission at physical examination and radiological expected Tumor size ≤ 2cm or non-mass enhancement ≤ 4cm at breast MRI after standard NACT; planned breast conserving surgery (BCS) with whole-breast irradiation; ECOG performance status 0-1. Exclusion criteria: SLNB before NACT; previous axillary surgery; bilateral BC. Patients who are eligible proceed BCS. After BCS, patients who showed BpCR are enrolled with omission of SLNB. Patients with no BpCR proceed with routine axillary surgery. The assumption for acceptable 5-year RFS ≥84% is based on previous study findings. The calculated total case number for per- protocol analysis is N=178. Primary endpoint: 5-year recurrence free survival. Secondary endpoint: local recurrence free survival, breast cancer specific survival, overall survival, ipsilateral axillary recurrence interval, distant metastasis free survival, contralateral breast free survival, re-operation rate according to breast biopsy after NACT, adverse event, and quality of life. The first patient was enrolled on September 2021. Among 147 patients who screened, 106 patients have been enrolled in January 2022. We plan to complete the target accrual by December 2025. Clinical trial information: NCT04993625.