We hypothesized that the more rapid elimination and lowerspleen uptake of [225Ac]Ac-DOTA-trastuzumab F(ab')2 and [225Ac]Ac-DOTA-trastuzumab Fab would makethese RIT agents less hematologically toxic but their effectivenessfor treating HER2-positive BC tumors in mice was not known, sincetheir more rapid elimination could decrease tumor uptake, diminishingtheir effectiveness for RIT. The gene discussed is ERBB2; the disease is breast cancer.