All Siamese twins are identical or 'monozygotic'.
Identical twins have the same appearance and sex because they are from a single egg and a single sperm that has divided into two embryos. However, when a developing embryo begins to split into identical twins but then stops part way, Siamese twins are formed. The failure of the embryo to split in two completely results in some part of the twins' bodies, such as the hip, chest, abdomen, buttocks, or head, being joined. Some of them may share internal organs such as a heart, brain, or liver. In some instances the individuals are joined only by a band of musculofibrous tissue and can be separated surgically.