Complex sentence: A large number of isoforms are caused by single nucleotide polymorphisms or SNPs, small genetic differences between alleles of the same gene; these occur at specific individual nucleotide positions within a gene.
Split into simpler sentences: A large number of isoforms are caused by single nucleotide polymorphisms or SNPs, small genetic differences between alleles of the same gene. These occur at specific individual nucleotide positions within a gene.