Human cells are kept healthy by the activity of millions of proteins. These proteins are modified in different ways, such as by adding sugar molecules to them, which can be crucial for them to function properly. Given this importance, defects in the sugar-adding process are often lethal at the very early stages of development. In rare cases, however, patients can develop sugar-adding deficiencies that result in a range of metabolic diseases, known collectively as 'congenital disorders of glycosylation' (CDG). These disorders are caused by defects in the enzymes involved in the sugar-adding process. For example, ALG2-CDG (or CDG-Ii) is a disorder caused by mutations in the ALG2 enzyme, which combines sugar molecules together. ALG2-CDG patients appear unaffected at birth, but later develop problems in different organs, such as the eyes, brain and muscles.