SMN1 and proximal spinal muscular atrophy: To investigate whether the lack of binding between WRAP53 and SMN was just a reflection of decreased levels of SMN in the SMA fibroblasts, or whether WRAP53 binding to the SMN protein derived from the SMN2 allele is in fact weaker, we quantified the relative amount of SMN that interacts with WRAP53 in SMA patients and in normal cells.