Corrigendum: Case Report: Two Monochorionic Twins With a Critically Different Course of Progressive Osseous Heteroplasia - Justicia-Grande, Antonio José; Gómez-Ríal, Jose; Rivero-Calle, Irene; Pischedda, Sara; Curras-Tuala, María José; Gómez-Carballa, Alberto; Cebey-López, Miriam; Pardo-Seco, Jacobo; Méndez-Gallart, Roberto; Fernández-Seara, María José; Salas, Antonio; Martinón-Torres, Federico (Frontiers in Pediatrics, 2021)