Mutations may bring about changes in the normal activity of the gene. These changes could lead to a gain or a loss of gene function. Hermann J. Muller, an American geneticist, classified mutations according to the effects of the mutation on the phenotype.1 Mutations leading to the complete elimination of the gene function are referred to as amorphic mutations. When there is only a partial loss of function, the mutation is called hypomorphic.
He also coined the term antimorph to refer to a mutation whereby the effect of the mutation leads to an altered gene activity that antagonizes the normal (wild-type) activity of the gene. Example of this is certain gene mutations implicated in cancer.
Other types of mutations include the neomorphic mutation and the hypermorphic mutation, which are similar to antimorphic mutation in a way that they are associated with mutations leading to a gain of gene function.
1Muller, H. J. 1932. Further studies on the nature and causes of gene mutations. Proceedings of the 6th International Congress of Genetics, pp. 213–255.