The formation of a normal plant or animal body structure or organ in place of another at an abnormal site. Examples of homeosis (also called homeotic transformation) are most obvious in insect appendages, where an appendage that is characteristic of one segment, for example the antennae on an insect head segment, are transformed into insect legs that normally develop only on trunk segments. Similar examples of homeotic transformations can also occasionally be found in vertebrates where lumbar vertebrae are transformed into thoracic vertebrae which then extend into rib processes, or in floral organs where petals are transformed into sepals. Homeotic transformations rarely occur in nature in living organisms, and are due to genetic defects in a class of proteins called homeotic proteins, the products of homeotic genes. Homeotic transformations may also be induced in the laboratory by the accidental or deliberate manipulation of homeotic gene expression so that homeotic proteins are produced in the wrong place or at the wrong time in developing plants and animals. See Cell differentiation, Developmental biology, Developmental genetics, Gene action, Mutation