A method of identifying a human subject as a carrier of a SMN1 duplication allele comprising: (a) screening a nucleic acid from a sample from a human subject by determining (i) the identity of the nucleotide corresponding to position 11678 of SEQ ID NO:1; or (ii) the identity of the nucleotide corresponding to position 15774 of SEQ ID NO:1; or (iii) the identity of the nucleotide corresponding to position 22804 of SEQ ID NO:1; or (iv) the identity of the nucleotide corresponding to position 26190 of SEQ ID NO:1; or (v) the identity of the nucleotide corresponding to position 27134 of SEQ ID NO:1, or (vi) whether the A-T dinucleotide corresponding to positions 27706-27707 of SEQ ID NO:1 has been deleted; and (b) identifying the human subject as a carrier of a SMN1 duplication allele if (i) the nucleotide corresponding to position 11678 of SEQ ID NO:1 is not G or there is a nucleotide inserted between nucleotides corresponding to positions 11678 and 11679 of SEQ ID NO:1; (ii) the nucleotide corresponding to position 15774 of SEQ ID NO:1 is not G or there is a nucleotide inserted between nucleotides corresponding to positions 15774 and 15775 of SEQ ID NO:1; (iii) the nucleotide corresponding to position 22804 of SEQ ID NO:1 is not G or there is a nucleotide inserted between nucleotides corresponding to positions 22804 and 22805 of SEQ ID NO:1; (iv) the nucleotide corresponding to position 26190 of SEQ ID NO:1 is not A or there is a nucleotide inserted between nucleotides corresponding to positions 26190 and 26191 of SEQ ID NO:1; (v) the nucleotide corresponding to position 27134 of SEQ ID NO:1 is not a T or there is at least one nucleotide inserted between nucleotides corresponding to positions 27134 and 27135 of SEQ ID NO:1; or (vi) the A-T dinucleotide corresponding to positions 27706-27707 of SEQ ID NO:1 is deleted. (a) screening a nucleic acid from a sample from a human subject by determining (i) the identity of the nucleotide corresponding to position 11678 of SEQ ID NO:1; or (ii) the identity of the nucleotide corresponding to position 15774 of SEQ ID NO:1; or (iii) the identity of the nucleotide corresponding to position 22804 of SEQ ID NO:1; or (iv) the identity of the nucleotide corresponding to position 26190 of SEQ ID NO:1; or (v) the identity of the nucleotide corresponding to position 27134 of SEQ ID NO:1, or (vi) whether the A-T dinucleotide corresponding to positions 27706-27707 of SEQ ID NO:1 has been deleted; and (i) the identity of the nucleotide corresponding to position 11678 of SEQ ID NO:1; or (ii) the identity of the nucleotide corresponding to position 15774 of SEQ ID NO:1; or (iii) the identity of the nucleotide corresponding to position 22804 of SEQ ID NO:1; or (iv) the identity of the nucleotide corresponding to position 26190 of SEQ ID NO:1; or (v) the identity of the nucleotide corresponding to position 27134 of SEQ ID NO:1, or (vi) whether the A-T dinucleotide corresponding to positions 27706-27707 of SEQ ID NO:1 has been deleted; and (b) identifying the human subject as a carrier of a SMN1 duplication allele if (i) the nucleotide corresponding to position 11678 of SEQ ID NO:1 is not G or there is a nucleotide inserted between nucleotides corresponding to positions 11678 and 11679 of SEQ ID NO:1; (ii) the nucleotide corresponding to position 15774 of SEQ ID NO:1 is not G or there is a nucleotide inserted between nucleotides corresponding to positions 15774 and 15775 of SEQ ID NO:1; (iii) the nucleotide corresponding to position 22804 of SEQ ID NO:1 is not G or there is a nucleotide inserted between nucleotides corresponding to positions 22804 and 22805 of SEQ ID NO:1; (iv) the nucleotide corresponding to position 26190 of SEQ ID NO:1 is not A or there is a nucleotide inserted between nucleotides corresponding to positions 26190 and 26191 of SEQ ID NO:1; (v) the nucleotide corresponding to position 27134 of SEQ ID NO:1 is not a T or there is at least one nucleotide inserted between nucleotides corresponding to positions 27134 and 27135 of SEQ ID NO:1; or (vi) the A-T dinucleotide corresponding to positions 27706-27707 of SEQ ID NO:1 is deleted. (i) the nucleotide corresponding to position 11678 of SEQ ID NO:1 is not G or there is a nucleotide inserted between nucleotides corresponding to positions 11678 and 11679 of SEQ ID NO:1; (ii) the nucleotide corresponding to position 15774 of SEQ ID NO:1 is not G or there is a nucleotide inserted between nucleotides corresponding to positions 15774 and 15775 of SEQ ID NO:1; (iii) the nucleotide corresponding to position 22804 of SEQ ID NO:1 is not G or there is a nucleotide inserted between nucleotides corresponding to positions 22804 and 22805 of SEQ ID NO:1; (iv) the nucleotide corresponding to position 26190 of SEQ ID NO:1 is not A or there is a nucleotide inserted between nucleotides corresponding to positions 26190 and 26191 of SEQ ID NO:1; (v) the nucleotide corresponding to position 27134 of SEQ ID NO:1 is not a T or there is at least one nucleotide inserted between nucleotides corresponding to positions 27134 and 27135 of SEQ ID NO:1; or (vi) the A-T dinucleotide corresponding to positions 27706-27707 of SEQ ID NO:1 is deleted.