Thrombocytopenia can lead to treatment delays, reduction in dose intensity and in some cases therapy discontinuation.1 T-DM1 and T-DXd are widely used in the treatment of breast cancers with HER2 expression.2-6 T-DM1 has received Food and Drug Administration (FDA) approvals for adjuvant HER2 positive breast cancer after prior neoadjuvant therapy with residual cancer and for late-stage HER2 positive breast cancer. The gene discussed is ERBB2; the disease is Thrombocytopenia.