Myb proteins suppress transformation by v-rel. Primary cultures of chicken embryo fibroblasts were co-transfected with the indicated proviruses and the MAV-1 helper virus. Two days later G418 (200 ug/ml) was applied to select for the expression of the neo gene. The morphology of the resulting G418-resistant cells was observed by phase contrast microscopy. The cells infected with the Rel-ER virus have a distinct transformed morphology in the presence but not in the absence of estrogen (10-6 M). This transformed morphology was suppressed by c-myb (Rel-ER-ICB) or v-myb (Rel-ER-IVB).