Surgeons have separated two-year-old Egyptian twins joined at the head in a painstaking operation at a Dallas, Texas, hospital.
The twins are undergoing a risky and lengthy operation
The operation on Mohamed and Ahmed Ibrahim, who have separate brains but share an important vein, began on Saturday and may last until Monday.
The boys faced a lifetime of medical problems if they remained connected.
The last such operation resulted in the deaths of adult Iranian twins in July but their father said he had no option.
Conjoined at the crown of the head, the boys have had trouble closing their eyes, moving their necks and swallowing.
On Sunday, surgeons at the Children's Medical Center in Dallas completed the most difficult and dangerous part of the procedure: separating the shared brain material and the shared circulatory systems which feed bloods to their brains.
"If there are no major complications, the operation should end on Sunday," said hospital spokesman Dr Jim Thomas.
"They are now within striking distance of living independent
The Egyptian twins' father told doctors to go ahead with the operation, saying he wanted to give them a chance at a normal life.
Born by Caesarean section in Qus, Egypt, 2 June 2001
Arrived in Dallas in June 2002
Tissue expanders inserted under skin in April 2003 to prepare for reconstruction
Op may take up to 48 hours
"If they're left this way, they're not going to be normal," said Ibrahim Mohammed Ibrahim earlier this year.
The World Craniofacial Foundation paid for the boys to come to the Children's Medical Center in Dallas when they were one-year-olds.
They have spent much of the past year having diagnostic tests done in preparation for surgery.
"I don't think anything could have been more studied than this has been," said Dr Dale Swift, one of four paediatric neurosurgeons performing the operation.
A team of 40 doctors, nurses and other staff led by top facial and cranial surgeon Dr Kenneth Salyer are taking part in the operation.
Doctors said earlier the brain material and large veins they shared could be divided without causing much harm but if their circulatory systems were not properly separated it could kill them.
However, they have youth on their side - with young bones and tissue that are more able to undergo the strain of the operation and recovery.
Relatives and friends are praying for the boys
The twins' uncle, Nasser Mohammed, said everyone in their home village of al-Homr, near the southern Egyptian city of Qus, were praying for them.
Mohammed and Ahmed are the first twins conjoined at the head to be surgically separated since the deaths of Iranian twins in July.
Laleh and Ladan Bijani, 29, died within 90 minutes of each other from massive blood loss during the separation surgery in Singapore in July.
Three of five operations on twins conjoined at the head, carried out in the last three years, have resulted in the survival of both children.
If the operation is successful, doctors will keep the twins in an artificially induced coma for another week to reduce brain swelling.