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Periodically the World Health Organization and currently the International Union of Immunology Societies publish a classification of primary immunodeficiency diseases (PID) that includes diagnostic and therapeutic guidelines. The latest of these publications dates from 1999 and includes a new group of PID, the proliferative autoimmune syndromes. Furthermore, new forms of severe combined immunodeficiency (SCID) and of recessive autosomal agammaglobulinemia are described. From the publication of this classification until the end of the year 2000 a minimum of three new PIDs have been described and a further two should probably be added. Progress in the molecular biology of these diseases has given rise not only to more accurate diagnosis but also to greater insight into the clinical spectrum of these diseases. A mutation or deletion in a gene can provoke the complete absence of its product; sometimes expression is partial or normal but functional activity is absent or defective. In certain cases, partial or defective activity causes variant forms of the disease presenting symptomatology or atypical cellular phenotype. In other cases, this is not cause of the variant form, which can appear in interfamilial cases sharing the same mutation. In these cases, these differences can be attributed to environmental factors or to other genes able to modify the affected gene. In this article we provide examples of variant forms in several PIDs. Some are late onset forms, such as X-linked agammaglobulinemias diagnosed in adults, since until diagnosis, clinical symptomatology was minimal. In adenosine-deaminase deficiency, a serious and highly lymphoproliferative form of SCID, patients have been described whose symptomatology began after the age of 20 years. Another SCID, RAG1 and RAG2 recombinase deficiency, may produce a typical form with a characteristic T-B-NK + phenotype, Omenn's syndrome, or forms with an unexpected T-B + NK + phenotype. Deficiency in common gamma chain receptor for IL-2 may produce phenotypical variants that can lead to diagnostic error. X-linked lymphoproliferative syndrome may present as fulminant infectious mononucleosis, as leukemia or lymphoma or as hipo- or agammaglobulinemia. Possibly, some patients diagnosed with common variable immunodeficiency or with x-linked agammaglobulinemia do in fact have this syndrome. Chronic granulomatous disease is usually of early-onset, but late-onset forms have been described. In one case the first clinical manifestation was produced when the patient was 60 years old. The above examples serve to highlight that, even though PIDs are usually suspected by pediatricians, in some cases the diagnosis may be missed by internists or non-pediatricians. Moreover, the clinical and laboratory findings of these variant forms must be determined to carry out an early diagnosis, which is essential for a favorable therapeutic outcome.