Family studies have led to identification of genes such as kinesin family member 21A (KIF21A; dominant congenital fibrosis of the extraocular muscles), paired-like homeobox 2a (PHOX2A; recessive congenital fibrosis of the extraocular muscles), roundabout homolog 3 (ROBO3; recessive horizontal gaze palsy with progressive scoliosis), and CHN1 (dominant Duane retraction syndrome) [5,6]. The gene discussed is KIF21A; the disease is Duane retraction syndrome.