Each gene produces exactly one protein.

The original one-gene–one-enzyme idea was a major conceptual breakthrough. It linked hereditary units to specific biochemical functions and helped turn genes from abstract Mendelian factors into molecular actors.

As molecular biology developed, the rule was refined to “one gene–one protein.” That still suggested a clean one-to-one mapping from a DNA sequence to a single final protein product.

Split genes and RNA processing disrupted the simplicity. In many eukaryotic genes, precursor RNA can be spliced in different ways, including or excluding particular exons. Different promoters, start sites, and processing choices can further diversify the resulting RNAs and proteins.

Some genes do not encode proteins at all; their functional products are RNAs.

The old rule remains useful pedagogically because genes often do have identifiable biochemical roles. But it is no longer a literal law of molecular biology.

Not every alternatively produced transcript becomes a stable or functional protein. The key correction is narrower: the mapping between genes and final products is not universally one-to-one.