[8] Chang and Eng Bunker (1811–1874) were brothers born in Siam (now Thailand) who traveled widely for many years and were known internationally as the Siamese Twins.

Over- or underexpression of this gene in laboratory animals results in severe malformations of the head region, including duplications, similar to some types of conjoined twins.

Surgery to separate conjoined twins may range from very easy to very difficult depending on the point of attachment and the internal parts that are shared.

Though there have been a number of successful separations throughout history, in many cases, the surgery results in the death of one or both of the twins, particularly if they are joined at the head or share a vital organ.

Alice Dreger of Northwestern University found the quality of life of twins who remain conjoined to be higher than is commonly supposed.

[24] In 1955, neurosurgeon Harold Voris (1902-1980)[25] and his team at Mercy Hospital in Chicago performed the first successful operation to separate craniopagus twins (conjoined at the head), which resulted in long-term survival for both.

[29] In 1957, Bertram Katz and his surgical team made international medical history performing the world's first successful separation of conjoined twins sharing a vital organ.

[30] Omphalopagus twins John Nelson and James Edward Freeman (Johnny and Jimmy) were born in Youngstown, Ohio, on April 27, 1956.

The 97-hour surgery on the pair of craniopagus twins was a landmark one which took place in Singapore; the team was led by neurosurgeons Chumpon Chan and Keith Goh.

Seven years later, Ganga Shrestha died at the Model Hospital in Kathmandu in July 2009, at the age of eight, three days after being admitted for treatment of a severe chest infection.

The operation was controversial because Rose, the weaker twin, would die as a result of the procedure as her heart and lungs were dependent upon Grace's.

Their complex case was accepted only because technologically advanced graphical imagery and modeling would allow the medical team to plan the risky surgery.

The surgery presented particular difficulties due to a number of shared veins and a distortion in the shape of the girls' brains, causing them to overlap.

"[41] According to Theophanes the Confessor, a Byzantine historian of the 9th century, around 385/386 AD, he writes that "in the village of Emmaus in Palestine, a child was born perfectly normal below the navel but divided above it, so that it had two chests and two heads, each possessing the senses.

Legend says that their father, Abd Manaf ibn Qusai, separated his conjoined sons with a sword and that some priests believed that the blood that had flowed between them signified wars between their progeny (confrontations did occur between Banu al'Abbas and Banu Ummaya ibn 'Abd Shams in the year 750 AH).

Other early conjoined twins to attain notice were the "Scottish brothers", allegedly of the dicephalus type, essentially two heads sharing the same body (1460–1488, although the dates vary); the pygopagus Helen and Judith of Szőny, Hungary (1701–1723), who enjoyed a brief career in music before being sent to live in a convent; and Rita and Cristina of Parodi of Sardinia, born in 1829.

Rita and Cristina were dicephalus tetrabrachius (one body with four arms) twins and although they died at only eight months of age, they gained much attention as a curiosity when their parents exhibited them in Paris.

Several sets of conjoined twins lived during the nineteenth century and made careers for themselves in the performing arts, though none achieved quite the same level of fame and fortune as Chang and Eng.