ERBB2 and cancer: To assess cellular binding, the free heavy chain cysteine of purified and activated Tfab was capped with fluorescein-5-Maleimide, and the fluorescent conjugate (Tfab-fluorescein-5-Maleimide) was assayed by flow cytometry with a panel of live human cancer cells exhibiting either high or low HER2 expression levels (S4 Fig).