IRX5 and obesity due to melanocortin 4 receptor deficiency: However, in the same study, mice homozygous for a new deletion mutation of Irx5 (Irx5Δ/Δ) exhibited early postnatal lethality, whereas heterozygous Irx5Δ/+ mutant mice were found to show an anti-obesity phenotype like Irx3 knockout mice.