Completed Phase I clinical trials of [99mTc]Tc-ADAPT6 (ClinicalTrials.gov ID: NCT03991260, https://clinicaltrials.gov/ct2/show/NCT03991260 (accessed on 8 June 2023)) and [99mTc]Tc-(HE)3-G3 (ClinicalTrials.gov ID: NCT04277338, https://clinicaltrials.gov/ct2/show/NCT04277338 (accessed on 8 June 2023)) in breast cancer patients with both positive and negative expression of HER2 have demonstrated that the injections of both proteins are well-tolerated, not associated with any adverse effects and result in a low radiation dose burden in patients. The gene discussed is ERBB2; the disease is breast cancer.