Midline-1 (Tripartite motif-containing protein 18, Putative transcription factor XPRF, RING finger protein 59) is a 667 amino acid protein encoded by the human gene MID1. Midline-1 belongs to the TRIM/RBCC family and contains two B box-type zinc fingers, one B30.2/SPRY domain, one COS domain, one fibronectin type-III domain and one RING-type zinc finger. Midline-1 is believed to have E3 ubiquitin ligase activity which targets the catalytic subunit of protein phosphatase 2 for degradation. It is a cytoplasmic protein found as a homodimer or heterodimer with Midline-2. It also interacts with IGBP1 (Lymphocyte signaling protein A4). Defects in MID1 are the cause of Opitz syndrome type I (OS-I). OS-I is an X-linked recessive disorder characterized by hypertelorism, genital-urinary defects such as hypospadias in males and splayed labia in females, lip-palate-laryngotracheal clefts, imperforate anus, developmental delay and congenital heart defects. OS-I mutations produce proteins with a decreased affinity for microtubules.