There are two alleles for this gene. You get one from your mom and one from your dad. In this case one allele has a mutation that is causative. So you inject some anti-sense RNA matched to that mutation, it will bind to the mRNA of the transcribed mutated gene DNA. Since it is now bound up, the mRNA cannot be translated by the ribosome and turned into protein. This means that only the other allele (without the mutation) is translated into protein.