A 2-month-old, breast-fed baby that was normal at bih begins to develop gastrointestinal problems and cirrhosis of the liver. Molecular analysis indicates a normal amount of galactose-1-phosphate uridyl transferase (GALT) mRNA, but no observable enzyme activity. Which of the following would be the best possible explanation for this?

A nonsense mutation is a mutation in the DNA that leads to a premature stop codon when the mRNA is being translated into protein. The mRNA would be transcribed correctly, but when the protein was being translated, it would be stopped prematurely, leading to a truncated protein. A gene deletion would result in either the absence of mRNA or mRNA of a different size, both of which would be apparent by Nohern blot analysis. A premature transcription termination sequence in the DNA might result in premature termination of the mRNA chain, leading to a diminished or absent message. A promoter mutation in the DNA, if it caused the gene in question to be less actively transcribed would result in a diminished amount of message.