Deletion of e.g. p63[30, 31] or Tbx3 (T-box gene 3) [32] may abolish the formation of all five MRs, while reduction of Wnt signaling by means of deletion of e.g. Lef1 (Lymphoid enhancer factor 1) [33] or Pygo2 (Pygopus 2) allows MR induction but leads to MR regression prior to nipple formation [34]. Here, TBX3 is linked to miotic rate.