Complex sentence: However, naturally occurring non-AUG start codons have been reported for some cellular mRNAs; and seven out of the nine possible single-nucleotide substitutions at the AUG start codon of dihydrofolate reductase were functional as translation start sites in mammalian cells.
Split into simpler sentences: However, naturally occurring non-AUG start codons have been reported for some cellular mRNAs. Seven out of the nine possible single-nucleotide substitutions at the AUG start codon of dihydrofolate reductase were functional as translation start sites in mammalian cells.