How many copies of SMN1 is normal?
How many copies of SMN1 is normal? Typically, people have two copies of the SMN1 gene and one to two copies of the SMN2 gene in each cell. However, the number of copies of the SMN2 gene varies, with some people having up to eight copies. What is the difference between SMN1 and SMN2? The two SMN genes are highly homologous and only differ at 5 base-pairs. These differences (in exons 7 and 8) are used to distinguish the telomeric (SMN1) from centromeric (SMN2) copy in DNA analysis. SMA of all types is associated with homozygous mutation (invariably deletions) of...