Background: Patients presenting with cT1-3N1M0 breast cancer often receive NACT to enable breast and/or axillary conservation. Targeted axillary dissection (TAD) (sentinel node biopsy (SNB) + removal of marked involved node) has been proposed to reduce the false negative rate of SNB after NACT in cN1 patients. In ATNEC, cT1-3N1M0 patients receive NACT followed by SNB/TAD. If the sentinel nodes (SN) have converted to benign (ypN0), patients are randomly assigned to Axillary Treatment vs no Axillary Treatment. Node marking is recommended in the study and sites are offered training to help them adopt node marking in a standardized way. Objectives: To prospectively evaluate the practice patterns and response to NACT, uptake of node marking, marked node identification rates and breast conservation rates in the ongoing UK ATNEC trial. The trial aims to recruit 1900 patients. Methods: Data for all patients who were randomized until 01-Feb-23 across 78 UK centers were included in these analyses. Patients were cT1-3N1M0 at presentation, received NACT and were found to have no residual nodal disease (ypN0) on SNB/TAD. Results: Among the 120 randomized patients (median age 54 [range 28-77] years) (median BMI 26.4 [range 17.6-51.1]), 53% (64) were post-menopausal, 78% (94) had breast conserving surgery (BCS) and 22% (26) mastectomy. At presentation, 14% (17) were T3, 64% (77) T2 and 20% (24) T1. 70% (84) had grade 3 tumors. 62% (74) were HER2 positive, 31% (37) triple negative (TNBC) and 9% (7) HER2 negative (ER or PgR positive). 54% (65) had an anthracycline and taxane based NACT and 36%(43) had platinum containing NACT. Median of 4 nodes [interquartile range 3-5] were removed during SNB/TAD. Of 109 patients with node marking data, the involved node was marked in 73% (80). The marked node was identified and removed in 95% (76/80), marked node was the SN in 88% (67/76). Of the 109 patients with data for breast tumor post NACT, 69% (75) had pathological complete response (pCR), 7% (8) DCIS only, 24% (26) invasive cancer. Of the 63 patients with complete breast tumor response on imaging, 17% (11) had residual DCIS/invasive cancer. 50% (21/42) of patients with partial response or stable breast tumor on imaging had no residual tumor on histology (ypT0). Of the 107 patients with data on post NACT axillary imaging, 14% (15) had partial or stable disease but pCR on histology. Conclusions: Majority of UK patients having NACT have HER2 positive or TNBC. Although 69% patients had a pCR (ypT0) in the breast, BCS rates remain low. Imaging has limited accuracy in predicting pCR after NACT. 3 in 4 patients randomized had their node marked, with a 95% intra-operative identification rate; demonstrating ATNEC has successfully rolled out node marking in the UK. Clinical trial information: NCT04109079.