MKI67 and breast cancer: Some examples are the variable “breast cancer subtype by proxy” to group patients with breast cancer according to hormone receptor, Ki67, and HER2 expression levels or “time interval between the end of the neoadjuvant treatment and surgery.” Additional difficulties include tumor grading systems such as the modified Ryan Scheme for Tumor Regression Grade, Miller and Payne’s Tumor Regression Grade, Residual Cancer Burden class, or grading of Ductal carcinoma in situ (DCIS).