Amino acid sequence alignment of the full-length and alternatively spliced FcμR along with the exon organization and the mass spectrometric results of serum FcμR. (Top) Exon organization of FCMR is schematically drawn to the scale indicated. Exons (boxes) encoding particular regions of the receptor are denoted as follows: 5′ untranslated (5′UTR), signal peptide (SS), extracellular Ig-like domain (Ig), stalk (Stalk), transmembrane (TM), cytoplasmic (CY), and the 3′ untranslated (3′UTR) regions. Roman numbers in each intron (horizontal lines) indicate the phase occurring between 2 exons. The lines connecting exon 4 (stalk 2) with exon 6 (CY1) indicate the alternative splicing identified in the cDNA of PMA-activated 697 pre-B cell line (gene accession HM480394). (Bottom) Amino acid sequences (single-letter code) deduced from the full-length (membrane-bound form; top mem.) and the alternatively spliced (bottom sol.) FcμR are aligned. Amino acid identity is indicated by dots (·) and a deletion by dashes (-). The unique 70 carboxyl-terminal residues of the FcμR splice variant are purple, and 2 predicted hydrophobic regions (SS and TM) are underlined. The sequences identified by mass spectrometric analysis of the tryptic peptide of the ∼ 40-kDa FcμR in CLL patients' sera are framed and highlighted in yellow. (B) Multiple reaction monitoring scan profile of one of the serum FcμR-derived peptide sequences. Liquid chromatographytandem mass spectrometric analysis reveals that this peptide eluted at a retention time of 24.21 minutes corresponds with the tryptic peptide “Q²³⁶SPLQAGPPTGR247” of the splice variant FcμR with a mass of 1190.6 Da, which covers the boundary of splicing of exons 4 and 6.