Experiments such as MEGA and SINDRUM have searched for lepton number violation in muon decays to electrons; MEG set the current branching limit of order 10 and plans to lower to limit to 10 after 2016. Some theories beyond the Standard Model, such as supersymmetry, predict branching ratios of order 10 to 10. The Mu2e experiment, in construction as of 2017, has a planned sensitivity of order 10.