Human MTHFR transcript variants.  The different transcripts (MTHFR1, 2, and 3) of the human MTHFR gene are determined by specific usage of the respective first exons.  The numbers underneath indicate the 5'-border of each exon or the end of the transcript.  The transcripts reveal individual open reading frames with unique start sites but constant stop codons. (Taken directly from Homberger et al, 2000)