IntroductionX-linked agammaglobulinemia (XLA) is the prototypic primary humoral immunodeficiency disorder, first described by Bruton who reported a boy with severe recurrent infections and absence of the gamma-globulin serum fraction. 1 This disorder has been of major interest for more than half a century, initiating an ongoing and fruitful search for the genetic basis of this and other primary immunodeficiency diseases.XLA is characterized by a lack of mature B cells and plasma cells and a profound deficiency of all immunoglobulin types. Most patients develop recurrent infections coincident with the loss of maternal antibodies. Pyogenic infection with encapsulated bacteria is the most common clinical manifestation. [2][3][4] Current therapy consists of regular immunoglobulin replacement and prompt attention to infection.Both XLA and a related murine immunodeficiency, X-linked immunodeficiency (Xid), result from deficiencies in Bruton tyrosine kinase (Btk). [5][6][7][8][9][10] Btk, a member of the Tec family of nonreceptor kinases, 11 is expressed throughout B-lineage development except in plasma cells. 12,13 Btk contains a Src homology 1 (SH1) catalytic domain, SH2 and SH3 protein interaction domains, and a unique amino-terminus with a pleckstrin homology (PH) domain. 14 Mutational studies indicate important functional roles for each of these domains. 15 The major developmental defect in XLA occurs at the pre-Bcell transition, resulting in an increase in pro-B cells and a marked reduction in cycling pre-B cells and all subsequent stages. [16][17][18] Few IgM ϩ B cells are detectable in the blood and the hypotrophic lymphoid tissues. 19,20 Clonal expansion at the pre-B stage is regulated by pre-B-cell receptor (pre-BCR) signaling and is essential for generating an adequate pool of immature B cells. 21 Pre-BCR engagement leads to the generation of a "signalosome" that includes activated Btk, 22 an event disrupted in XLA.Several murine models of Btk deficiency currently exist. Xid mice have a spontaneous missense mutation in the PH domain. 23 The mutant protein is inefficiently recruited to the plasma membrane and fails to enter the BCR signalosome. 24 The phenotype of both Xid mice and Btk null-knockout mice (Btk Ϫ/Ϫ ) is less severe than that of XLA. [25][26][27][28] These mice produce nearly normal numbers of peripheral B cells, but splenic B-cell development is significantly compromised. Btk-deficient transitional 2 (T2) immature B cells fail to generate the BCR-dependent pro-survival, proliferative, and differentiation signals required to produce mature B An Inside Blood analysis of this article appears in the front of this issue.Reprints: David J. Rawlings, Children's Hospital and Regional Medical Center, 307 Westlake Ave North, Suite 300, Seattle, WA 98109; e-mail: drawling@u.washington.edu.The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked ''advertisement'' in accordance with 18 U.S.C. section 173...