April 29 (Reuters) - X4 Pharmaceuticals ( XFOR ) said on

Monday the U.S. FDA has approved its therapy to treat a rare

genetic immunodeficiency disease in patients 12 years of age and

older, sending its shares up 16% before the bell.

X4's mavorixafor, which will be sold under the brand name

Xolremdi, is the first therapy to get U.S. approval specifically

for the treatment of WHIM syndrome.

WHIM is characterized by disorders in which the body's

immune system does not function properly. It is an acronym for

warts, hypogammaglobulinemia or low antibody levels, infections,

and myelokathexis or low white blood cell count, which are all

symptoms of the disorder.

The decision is based on a late-stage study where the

therapy showed 60% reduction in infection rate compared to

placebo when tested in patients with WHIM syndrome.

Less than 1,000 people in the United States have WHIM

syndrome, according to government data.