The 5'-UTR splicing variants of MS4A1 (encoding CD20) and their frequency in mature B-cell malignancies. (A) The transcription of MS4A1 is regulated by multiple cell signals and transcription factors (TFs). Four 5'-UTR isoforms (V1-V4) of MS4A1 have been identified resulting from distinct splicing events between exon 1 and 3 (internal splice site), but all have identical coding sequences (translation start codon is localized within exon 3). The V3 and V4 represent translation proficient variants, whereas inefficient translation of V1 and V2 transcripts is rate-limiting for CD20 protein production. The V1 and V2 variants harbor multiple upstream open reading frames and a stem-loop secondary structure containing binding sites for Sam68 protein. (B) The frequency of V1-4 variants of MS4A1 in samples from multiple cohorts of B-cell malignancies (FL, n = 2; endemic BL, n = 3; DLBCL, n = 4; CLL, n = 3). The V1 and V3 represent together >90% of MS4A1 isoforms, and their ratio has a wide distribution within samples from the same diagnosis. Professional illustration by Patrick Lane, ScEYEnce Studios.