The word “isoforms” has two distinct but related meanings. If you have linked to this page from another page, then the word isoform is probably being used to refer to two different proteins coded for by a single gene. This is the result of alternative splicing of messenger RNA (mRNA). Eukaryote genes are a sequence of exons and introns coded as a sequence of nucleotide base pairs of DNA. Before a gene can be expressed as a protein, that gene must be transcribed into mRNA. Introns are then spliced out of the mRNA before the mRNA is translated into protein. If the mRNA can be spliced in different ways, or if the mRNA is translated before it is spliced, then it gives rise to different proteins. As a consequence of alternative splicing, a single gene can give rise to both soluble proteins and membrane bound proteins.
See w:isoform for alternative meanings of the word “isoform”.