Four brothers in the U.K. have all been stricken with a rare, often fatal, genetic disorder, according to a BBC report.

Luke Hartley, 7, and his brothers Daniel, 11, Nathan, 13, and Joshua, 15, from Romsey, Hampshire in the U.K., were all born with the immune system disorder XLP — short for X-linked lymphoproliferative syndrome or Duncan's syndrome.

It is one of the rarest fatal genetic disorders in the world and is believed to affect only about 100 families, according to BBC.

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Luke just returned home after a two-month stay over Christmas at Great Ormond Street Hospital in London, where he was undergoing a second bone marrow transplant. The first transplant the boy received failed. He is the last of the four to receive a transplant.

The brothers were diagnosed in November 2003 with XLP. Their parents, David and Allison Hartley, were told the boys were unlikely to reach their teenage years without bone marrow transplants.