 the β chain ITAM (Nunomura et al., [@B34]).

We think that this type of β chain-induced enhancement may play a crucial role in allergic reactions occurring *in vivo*, such as in bronchial asthma. The IgE-Fc~ε~RI-mast cell system was shown to be critically involved in an oxazolone-induced contact hypersensitivity (CHS) mouse model (Kobayashi et al., [@B21]). The role of the β ITAM has also been examined using this mouse model.

In this review we have mainly focused on recent findings regarding the roles of the FcεRIβ chain, especially the dual (positive and negative) regulatory roles of the FcεRIβ chain ITAM, both *in vitro* and *in vivo*. Detailed reviews on FcεRI signaling, including this topic, are available (Kraft et al., [@B23]; Rivera and Gilfillan, [@B39]; Kraft and Kinet, [@B22]; Rivera et al., [@B38]). Findings regarding the novel roles of the FcεRIβ chain in the fine-tuning of mast cell activation will contribute to investigation in new areas for the development of therapeutic interventions for allergic diseases.

Structure of the FcεRIβ-Chain
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The cDNA for the FcεRIβ chain was identified from a cDNA library derived from a rat mucosal mast cell tumor in 1988 (Kinet et al., [@B20]). Subsequent studies identified mouse and human FcεRIβ chain counterparts (Blank et al., [@B2]; Ra et al., [@B37]; Küster et al., [@B24]). The human and mouse FcεRIβ chain genes contain seven exons. The start and stop codons are located in exon 1 and exon 7, respectively (Figure [1](#F1){ref-type="fig"}A). The homology among the amino acid sequences of the rat, mouse, and human β chain proteins is approximately 69% (Küster et al., [@B24]).

![**FcεRIβ gene encoding the full-length form of the protein (β) and two truncated alternative splicing products (βT and MS4A2~truc~)**. **(A)** FcεRIβ gene structure. **(B)** Transcript structure. **(C)** FcεRIβ protein structure. βT and MS4A2~truc~ lack the C-terminal cytoplasmic region and transmembrane (TM) region (dashed line)