Holoprosencephaly. Coloured magnetic resonance imaging (MRI) scan of a brain (green) with holoprosencephaly. In this congenital condition, a large abnormal ventricle (fluid-filled cavity, pink/blue) forms where the forebrain would norma- lly be. Two healthy ventricles (blue/pink) are at lower left and right. People with this deformity often have no olfactory system (smell), as well as defects in the optic nerve, eyes and pituitary gland. A range of facial deformities reflect the severity of the brain defect, as does the degree of mental handicap. The condition is caused by trisomy, the inheritance of a third chromosome, in this case either chromosome 13, 14, 15 or 18.