See all Craniofacial services What is Pierre Robin Sequence? Pierre Robin Sequence (PRS) is the name of a birth condition that results when the lower jaw fails to develop normally during the first trimester. The lower jaw may be micrognathic (small in size) or retrognathic (set back from the upper jaw). The abnormal size or position of the mandible causes the tongue to be displaced posteriorly into the throat (glossoptosis).