ERBB2 and breast cancer: To check whether anti-Id scFv vaccination could protect MMTV.f.huHER2(Fo5) females from developing mammary tumors after the physiological changes linked to pregnancy, we carried out a second experiment in which females were immunized with PBS alone (n = 6), scFv40 (n = 9), scFv69 (n = 7) or HER2-Fc (n = 6) according to the usual protocol and then underwent pregnancy.