Changes or mutations at the DNA level could result in a different protein than what it was originally coded for. You can think about this question by referring to the central dogma of biology:
DNA -> mRNA (transcription), mRNA -> protein (translation)
A change of DNA would lead to a change in mRNA and if it is a change that matters (etc. leads to a premature stop codon), the protein that the mRNA codes for could end up different or non-functional.