Pierre Robin sequence is caused by defect in development of which pharyngeal arch?

Robin sequence may occur independently or in association with other syndromes and malformations. Like Treacher Collins syndrome, Robin sequence alters first-arch structures, with development of the mandible most severely affected. Infants usually have a triad of micrognathia, cleft palate, and glossoptosis (posteriorly placed tongue) Ref: Langman's embryology 11th edition Chapter 16.