Lateral Meningocele or Lehman Syndrome (LMS) is associated with NOTCH3 mutations causing deletions of the PEST domain and a gain-of-NOTCH3 function. We demonstrated that Notch3^em1Ecan mice harboring Notch3 mutations analogous to those found in LMS are osteopenic because of enhanced bone resorption. To determine the contribution of specific cell lineages to the phenotype, we created a conditional-by-inversion (Notch3^COIN) model termed Notch3^em2Ecan in which Cre recombination generates a Notch3^INV allele expressing a NOTCH3 mutant lacking the PEST domain. Germ line Notch3^COIN inversion caused osteopenia and phenocopied the Notch3^em1Ecan mutant, validating the model. To induce the mutation in osteocytes, smooth muscle and endothelial cells, Notch3^COIN mice were bred with mice expressing Cre from the Dmp1, Sm22a and Cdh5 promoters, respectively, creating experimental mice harboring Notch3^INV alleles in Cre-expressing cells and control litter-mates harboring Notch3^COIN alleles. Notch3^COIN inversion in osteocytes led to femoral and vertebral cancellous bone osteopenia, whereas Notch3^COIN inversion in mural Sm22a or endothelial Cdh5-expressing cells did not result in a skeletal phenotype. In conclusion, introduction of the LMS mutation in osteocytes but not in vascular cells causes osteopenia and phenocopies Notch3^em1Ecan global mutant mice.