It is part of the selenoprotein family. Selenoproteins are mainly involved in chemical reactions called oxidation-reduction reactions, which are necessary to protect cells from damage they cause.
In babies / children born with the SEPN1 mutation, the first symptoms are probably poor muscle tone and / or general laxity with delayed motor skills such as crawling or walking. Muscle weakness is most likely confined to the trunk and neck first.
Affected individuals may have difficulty raising their heads. Walking is regained, but can become difficult due to spinal stiffness and rapidly developing scoliosis. Respiratory failure can develop in the first decade of life even when the affected person is still standing, leading to the need for use of non-invasive ventilation at night. Affected individuals are usually short and have difficulty gaining weight.