ABSTRCT Purpose To explore the evolution of HER2 nega-tive subgroups(IHC Null,Ultra-low and 1+)in breast cancer with hormone receptor(HR)positive before and after neoadju-vant therapy,and the relationship with clinical pathological fea-tures.Methods There were 255 patients who did not achieve pathological complete response(pCR)consecutively after neoad-juvant therapy.Immunohistochemistry was used to detect the ex-pression of ER,PR,HER2 and Ki67 and to evaluate the evolu-tion of HER2-negative subgroups after neoadjuvant therapy and its relationship with clinicopathological characteristics.Results Among the 255 patients included in this study,HER2 expression was 0 and 1+in 116 cases(45.5％)and 139 cases(54.7％)respectively before neoadjuvant therapy,and then HER2 0 was further divided into Null group(61 cases,23.9％)and Ultra-low group(55 cases,21.6％).After neoadjuvant therapy,HER2 expression was 0 and 1+in 117 cases(45.9％)and 138 cases(54.1％)respectively,and then HER2 0 was further di-vided into Null group(64 cases,25.1％)and Ultra-low group(53 cases,20.8％).HER2 status changed in 121 patients(47.5％)after neoadjuvant therapy.The highest conversion rate was from HER2 Ultra-low before neoadjuvant therapy to 1+after neoadjuvant therapy,with a conversion rate of 11.76％(30/255),followed by HER2 1+to the Ultra-low,with a conversion rate of 10.98％(28/255).After the neoadjuvant therapy,44 of 55 cases had transformation in the HER2 Ultra-low group,with the conversion rate of as high as 80％.Chi-square test showed that HER2 expression before neoadjuvant therapy was correlated with the maximum tumor diameter(≤2 cm,＞2cm)after neo-adjuvant therapy(x2=6.106,P=0.047);the tumor of HER21+before neoadjuvant therapy was mostly 2 cm or less in the di-ameter.The HER2 status after neoadjuvant therapy was correla-ted with the tumor thrombus(x2=6.975,P=0.029).Patients with HER2 Ultra-low after treatment were more likely to have vascular invasion.Conclusion In HR positive breast cancer,when the HER2 0 cases are divided into Ultra-low and Null sub-groups,the HER2 conversion rate increases significantly after neoadjuvant therapy,in which the Ultra-low conversion rate is the highest,indicating that the HER2 Ultra-low cases are highly unstable after neoadjuvant therapy.It is important to detect HER2 expression in residual lesions after neoadjuvant therapy and to identify the Ultra-low HER2 expression subgroup.