The gene encoding the gamma chain of the lymphocyte interleukin-2 receptor has been cloned and shown to be required to associate with the beta chain in order for IL-2 internalization and cell activation to occur (1). We considered this gene, IL2RG, a candidate for the X-linked form of severe combined immunodeficiency at the SCIDX1 locus, in which affected males have impaired lymphocyte development. Using fluorescence in situ hybridization and PCR amplification of somatic cell hybrid DNAs, we mapped IL2RG to human Xq13.1, a location within the SCIDX1 critical region established by linkage analysis. The 4.2 kb IL2RG gene was sequenced, and its genomic organization was elucidated. Seven of 19 transformed B-lymphocyte cell lines with independent SCIDX1 mutations had absent or minimal IL2RG mRNA. Unique point mutations were documented to be specifically associated with the disease and the carrier state in four unrelated affected males and their family members: one in a boy with no detectable IL2RG mRNA, in which the mutation ablated a splice donor site; one causing premature chain termination; and two causing distinct amino acid changes. The demonstration of impaired IL2RG mRNA expression in males with X-linked SCID and of unique point mutations in SCIDX1 pedigrees constitutes powerful evidence that the SCIDX1 gene is IL2RG. Noguchi et al. (2) have independently published IL2RG mapping to Xq13 and discovery of mutations in three affected males. The specific pathogenesis of IL2RG mutations and approaches to gene therapy can now be addressed in the X-linked form of SCID.

Mixed infections with seasonal influenza A virus strains are a common occurrence and an important source of genetic diversity. Prolonged viral shedding, as observed in immunocompromised individuals, can lead to mutational accumulation over extended periods. Recently, drug resistance was reported in immunosuppressed patients infected with the 2009 pandemic influenza A (H1N1) virus within a few days after oseltamivir treatment was initiated. To better understand the evolution and emergence of drug resistance in these circumstances, we used a deep sequencing approach to survey the viral population from an immunosuppressed patient infected with H1N1/2009 influenza and treated with neuraminidase inhibitors. This patient harbored 3 genetic variants from 2 phylogenetically distinct viral clades of pandemic H1N1/2009, strongly suggestive of mixed infection. Strikingly, one of these variants also developed drug resistance de novo in response to oseltamivir treatment. Immunocompromised individuals may, therefore, constitute an important source of genetic and phenotypic diversity, both through mixed infection and de novo mutation.

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