Background: In the ongoing neoadjuvant ATNEC breast cancer trial (NCT04109079), cT1-3N1M0 patients receive neoadjuvant chemotherapy (NACT) followed by sentinel node biopsy (SNB) or targeted axillary dissection (TAD). If the sentinel nodes (SNs) convert to benign (ypN0), patients are randomized to receive Axillary Treatment vs. no Axillary Treatment. Standardized node marking procedures are promoted in this study. This study evaluates node marking techniques, rates of marked node identification, and concordance of the marked node with the sentinel node in the initial 500 participants. Methods: Data from 587 patients registered from December 2021 to January 1, 2024, across 77 UK centers were analyzed. These patients presented with cT1-3N1M0 breast cancer and received or planned to receive NACT. Results: The median age was 54 years (range 28-81), with a median of 4 nodes [interquartile range, 3-5] removed during SNB/TAD. Among 470 patients with node marking data, 72% (339) had the involved node marked. Sentinel node biopsy was performed using a combination of radioisotope and blue dye in 51%, blue dye only in 13%, radioisotope only in 10%, and Magtrace only in 9%. The marked node was successfully removed in 90% (305/339) of these patients, with the marked node being the sentinel node in 80% (243/305) of cases. In patients with complete pathological nodal response, the marked node did not show signs of downstaging (e.g. fibrosis) in 15% (46). Common techniques used for node marking included clip/coil only before NACT (29%), clip/coil before NACT with another wireless marker after NACT (26%), Magseed only before NACT (16%), and black dye only before NACT (15%). Conclusions: Notably, 72% of patients underwent node marking, with an intra-operative identification rate of 90%, demonstrating the successful implementation of node marking in the UK through the ATNEC trial. Clinical trial information: NCT04109079.