Mutations in a number of other Wnt ligands or receptors expressed in the fetal foregut, including Wnt4, Wnt5a, Wnt7b, Ror2 and Rspo2, also display deficits in cartilage development with varying extents of tracheomalacia (Bell et al., 2008; Caprioli et al., 2015; Kishimoto et al., 2018; Li et al., 2002). This evidence concerns the gene RSPO2 and Tracheomalacia.