B6-3*hSMN2 mice are a humanized disease model carrying three copies of the human SMN2 gene, which can be used to mimic SMA patients with three SMN2 gene copies. Since the SMN2 gene primarily produces SMNΔ7 protein lacking exon 7, rather than full-length SMN protein, the humanized SMN2 gene cannot fully compensate for the abnormalities caused by Smn1 deficiency, resulting in the manifestation of SMA-like phenotypes in this model.