Gene expression is where the mRNA is made. If there's an error in transcribing the DNA into mRNA there could be a major effect on consequent gene expression if:
1. the original gene is involved in gene expression itself (e.g. the RNA polymerase)...if it's not made correctly you would have serious future problems.
2. if the original gene is for a protein involved in further gene expression (e.g. transcription factor/regulator) - if made incorrectly it may not be there to properly control gene expression so you could get other genes expressed when they shouldn't be, or not expressed when they should be.