The father (II:2) of the affected boy (III:2) was found to be heterozygous for the deletion, whereas the 45 year old mother (II:1) carried a biallelic deletion of exon 7 and 8 of SMN1. Quantification of SMN2 gene copy number in the members of family 1 allowed us to explain the absence of SMA symptoms in the mother. Here, SMN1 is linked to proximal spinal muscular atrophy.