Point mutations

Most disease-causing mutations are simple base substitutions,
which can have variable effect. Mis-sense mutations result in the
replacement of one amino acid with another in the protein
product and have an effect when an essential amino acid is
involved. Non-sense mutations result in replacement of an
amino acid codon with a stop codon. This often results in
mRNA instability, so that no protein product is produced.
Other single base substitutions may alter the splicing of exons
and introns, or affect sequences involved in regulating gene
expression such as gene promoters or polyadenylation sites.