Human BCL11A locus and predicted protein isoforms. Alternative splicing within the BCL11A locus (Entrez Gene ID 53335) leads to the creation of four major protein isoforms: eXtra Long (XL; accession AJ404611), Long (L; accession AJ404612), Short (S; accession AJ404613), and eXtra Short (XS; accession AY692278). Exons 1 and 2 are common to all four isoforms, which encode a superfamily repression domain and also an unusual C2HC zinc finger likely to be involved in protein-protein interactions (see Discussion). Unlike the XS isoform, XL, L and S all employ at least a portion of Exon 4 (which encodes all six Krüppel-type C2H2 zinc fingers). Also shown are the chromosomal breakpoints , 1.5 kb upstream of Exon 1, that first were described in pediatric B-CLL patients AS and LH who bore the reciprocal translocation t(2;14) (p16.1;q32.3).