BTK and acute lymphoblastic leukemia: By contrast, primary leukemic cells from an infant B-cell precursor ALL patient with very low protein levels of BTK and aberrant BTK transcripts characterized by deletion of exon 16 that results in a truncating frameshift mutation starting at amino acid #523 within the catalytic domain with 4 novel amino acids before the stop codon [15] showed an aberrant, predominantly cytoplasmic localization of IK (Figure 2A).