For the first time, clinicians have successfully treated a fetus in utero with enzyme replacement therapy (ERT). A team of doctors and researchers from the University of California San Francisco, Duke and the University of Ottawa treated an unborn child with Pompe disease, a rare autosomal recessive metabolic disorder. The disease is caused by deficiency of the lysosomal acid α-glucosidase enzyme, which leads to glycogen aggregates in tissues and is especially damaging to cardiac and skeletal muscle. Although newborn screening allows ERT with recombinant α-glucosidase to start soon after birth, which improves outcomes, in the severe infantile onset form of the disease, the damage begins in utero.