Phosphomannomutase 2 deficiency (PMM2-CDG) is the most common congenital disorder of glycosylation (Ferreira et al. 2018), and was originally described in the literature as CDG1a/CDG1/Jaeken Syndrome (Matthijs et al. 1997). The gene discussed is PMM2; the disease is PMM2-congenital disorder of glycosylation.