The new MMTV-tTA strain exhibits transgene activation in the perinatal mammary gland.

<p><b>A</b>. GFP expression in MMTV-tTA/TetO-H2B-GFP double transgenic embryos on days E12.5 and E14.5. Bright-field (left) and GFP (right) images were taken on a fluorescent stereoscope equipped with a Spotflex camera. Sg, salivary gland; arrows indicate the location of the mammary placodes <b>B.</b> Expression of GFP in a newborn MMTV-tTA/TetO-H2B-GFP double transgenic female pup (a, b, e, f, i, j), a female TetO-H2B-GFP single transgenic negative control (c, d, g, h), as well as a male double transgenic littermate (k, l). The panels i. and j. show a higher magnification of the bright-field and GFP images of the nipple regions (arrows) in panels e. and f. Note that male pups lack mammary glands and, therefore, expression of GFP, yet exhibit strong activation of the reporter transgene in the salivary gland (Sg; panels k. and l.).