Isolation and Culture of Tumor Cells

Cells were maintained in DMEM + 10% fetal bovine serum (FBS, Axenia Biologix, Dixon, CA USA) and penicillin/streptomycin (Gibco, Waltham, MA USA) at 37°C under 10% CO₂ and were were routinely passaged using 0.05% typsin-EDTA (Gibco). Primary NDL tumor cells were dissociated by enzymatic and mechanical methods essentially as described for normal mammary tissue^{42 (link)} from Muc4^wt/NDL (n=4) and Muc4^ko/NDL (n=4) tumors. Tumor cells were harvested from the most advanced tumor and used within four passages to ensure equal comparison. The expression of Muc4 was retained over this duration, as confirmed by immunocytochemistry (data not shown).