A point mutation is the most limited type of mutation, whereby a single base nucleotide is replaced with another nucleotide. Sometimes the expression "point mutation" can also include the addition or subtraction of a single base pair.
The sickle-cell disease has been described as a single point mutation in the beta hemoglobin gene, whereby a GAG codon is converted into GTG. The GTG then mistakenly encodes the amino acid valine instead of glutamic acid.