ESR1 and breast cancer: The large number of CK7/Hoechst-positive cells with or without ER expression on the first CellSieve device (from blood tube A) and presence of CK7/Hoechst-positive cells but no CK20 (and possibly no TTF-1) on the second CellSieve device (from blood tube B) of the patient after both on-study blood draws is consistent with the patient's known primary breast cancer.