Background: In the NA-PHER2 study we assessed the association between biological pathways with pathological complete response (pCR) and Ki67 down-regulation Methods: Patients with centrally confirmed ER+ ( > 10%) HER2+ breast cancer (BC) were treated in two independent, non-randomized cohorts with neoadjuvant trastuzumab, pertuzumab, palbocilib with (Fulv, n = 30) or without (NoFulv, n = 28) fulvestrant (+/-LHRH analogues). We assessed RNA-seq on core-biopsies obtained pre-treatment [n = 53/58 (91.4%)], at day 14 [n = 49/58 (84.5%)], and on residual disease at surgery [n = 42/45 (93.3%)]. We investigated biomarker dynamics and association with pCR or Ki67 down-regulation (centrally evaluated) at day 14 and at surgery. In the overall population and in each cohort, we primarily assessed three pre-defined biomarkers (ER-metagene [from OncotypeDX], a CD8-metagene and ERBB2 expression), and secondarily we explored a pre-defined list of genesets. Continuous and categorical (median cut-point) variables were evaluated. Results: In the biomarker population, pCR rate was 22.5% (28.6% and 16.0% in Fulv and NoFulv cohorts). At baseline, continuous CD8-metagene (OR 1.85 [1.12-3.06], p = 0.016) and ER-metagene (OR 0.56 [0.34-0.90], p = 0.016) associated with higher and lower pCR rate, respectively. High ERBB2 (above median) was marginally associated with pCR (OR 3.83 [0.90-16.3], p = 0.068). Only ER- and CD8-metagenes retained significance in multivariate analysis and were similarly predictive in both cohorts. Combining categorical variables, the groups with high-CD8/low-ER and low-CD8/high-ER had 61.5% and 0% pCR rate respectively, whereas low-CD8/low-ER and high-CD8/high-ER had similar 15% pCR (p = 0.001). The association was significant in both cohorts (p = 0.019 Fulv; p = 0.028 NoFulv). Dynamic assessment of the same biomarkers at day 14 did not improve prediction. Higher ER-metagene at baseline, but not CD8 and ERBB2, was associated with robust down-regulation of Ki67 at day 14 (Ki67 < 2.7%, complete cell cycle arrest) only in Fulv cohort (p = 0.016). ER-metagene also associated with retained Ki67 down-regulation (Ki67 < 10%) at surgery (p = 0.002). Alternative ER- and immune-related signatures provided very similar results. The comprehensive landscape of complex molecular dynamics and exploratory association with outcome will be presented. Conclusions: In ER+/HER2+ BC, low expression of ER-related and high expression of immune-related genes identified patients with very high likelihood of achieving pCR with a chemo-free regimen. In the fulvestrant cohort, the group with high ER-metagene, despite a lower pCR rate, had higher Ki67 down-regulation at day 14, which has been associated with long-term benefit in luminal tumors. These findings provide a potential tool for tailored de-escalation strategies.