WapInt6sh relative expression levels determined by linear RT-PCR. The top panel shows the relative expression of Int6sh in various wild-type and transgenic mammary glands at different stages of development. Expression of the housekeeping gene, GAPDH is shown for comparison. The level of expression of Int6sh in lanes 6–9 is proportional to the extent of hyperplasia observed in those glands. The bottom panel shows Int6sh expression from dissected portions of the mammary glands of one mouse (no. 1,581, lanes 10–12), from independently arising tumors (lanes 13–16) and from outgrowths arising after transplanting tumor fragments (lanes 17,18). Histologically 'normal' mammary tissue (shown in lanes 10, a and b) was dissected away from surrounding focal hyperplasias and tumor tissue and then pooled. Transplantation of parous WapInt6sh tumor fragments into mammary epithelium-divested fat pads produced outgrowths that expressed detectable amounts of Int6sh RNA, suggesting that they also arose from mammary cells constitutively expressing the transgene (lanes 17,18). First strand reactions without RT and those with RT are designated as a and b respectively.