These Ror1f/f mutant mice possess loxP sites flanking exons 3-4 of the receptor tyrosine kinase-like orphan receptor 1 (Ror1) gene. Mice that are homozygous for this allele are viable and fertile. ROR1 is a receptor for Wnt5a-mediated signaling. When bred to mice that express tissue-specific Cre recombinase, resulting offspring will have exons 3-4 deleted in the cre-expressing tissues. When bred to Tg(EIIa-cre)C5379Lmgd transgenic mice, ROR1 null mice are morphologically and phenotypically normal, although some exencephaly has been seen. When Ror2 gene expression is also abolished (see Stock No. 018354) these double KO mice exhibit decreased branching of the sympathetic neuron axons, facial malformations, and truncation of the limbs and posterior region. Double KO mice die by E14.5. The Jackson Laboratory 2024, used with permission.