Video 1.

Video is available through the following link: https://www.dropbox.com/s/k1y5i2pj857k993/Supplementary%20video.mp4?dl=0/. Segment 1 shows the proband (III‐5) with generalized dystonia, spasticity, and knee/elbow joint contractures. Segment 2 shows the proband sitting leaning towards the wall. She has dystonia in her fingers and the left foot. Segment 3 shows dystonia, increased muscle tone, and severe contractures at the elbows and knees in the proband. Segment 4 Shows increased tendon reflexes from the upper limbs in the proband. She does not follow the hummer with her eyes when checking pursuit. Segment 5 shows that proband cannot stand and walk. Segment 6 shows upward tonic eye deviation in the affected sibling, very subtle twitching of the perioral muscles, and one myoclonic head jerk (III‐6). Segment 7 shows the inability to sit independently, axial hypotonia, and no head control in the affected sibling. Periodic myoclonic jerks are also visible in the upper limbs (left more than right) together with dyskinetic movements in all limbs. Segment 8 shows limb dystonia, increased muscle tone, and brisk tendon reflexes in the affected sibling.