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Pedigrees of spinal muscular atrophy (SMA) families with biallelic absence of the SMN1 gene in unaffected subjects. (a) family 1, (b) family 2; circles represent females, squares represent males, black symbols represent biallelic SMN1 absence in affected individuals, dot symbols represent heterozygous SMA carriers, vertical line symbols represent biallelic SMN1 absence in unaffected individuals.
