[Inborn errors of metabolism as rare diseases with a specific global situation]

So-called congenital metabolic diseases (CMD) are a consequence of biochemical alterations originating in the genes that result in the alteration of a protein. Depending on this protein's function - whether as an enzyme, a hormone, a receiver-transporter of a cellular membrane or forming part of a cellular organelle (lysosome, peroxysome) - different groups of diseases emerge, which cause the most outstanding characteristic of inborn errors of metabolism (IEM): their clinical heterogeneity. The majority of these diseases are autosomal recessive, with a limited number of asymptomatic carriers, but there are also those ruled by an autonomous, dominant character inheritance or linked to the X chromosome. Taken individually, CMDs are highly infrequent, but taken as a whole CMDs (of which over 500 have been described to date) can affect 1/500 of the newborn. A common characteristic of many CMDs is the possibility of dietary treatment and treatment with enzymatic replacement. For essentially didactic purposes the following groups should be considered: CMDs of the intermediary metabolism (whose types are intoxication and energy deficit), CMDs of cellular organelles, complex CMDs due to cycle alterations and others. A summary is presented of the clinical, diagnostic and therapeutic aspects of one disease of each type of those previously described: hyperphenylalaninemias, deficiencies of the mitochondrial oxidative phosphorilation (OXPHOS) and lysosomal storage diseases.