ERBB2 and breast cancer: According to ClinicalTrial.gov, more than 100 trials on breast cancer brain metastases are now in progress, including studies focusing on new agents (cyclin-dependent kinase (CDK) inhibitors, poly (ADP-ribose) polymerase (PARP) inhibitors, phosphoinositide 3-kinase (PI3K) inhibitors, new generation tyrosine kinase inhibitors (TKIs), anti-HER2 conjugates, immune drugs) with the evaluation of the objective intracranial response rate as one of the study endpoints [9].