Enu mutagenesis caused an A to G transition at nucleotide 864 in exon 8 (GenBank accession number M57696). This converted amino acid 260 from a glutamic acid into a glycine (E260G). This amino acid is immediately C terminal to the GxGxxG G loop necessary for kinase activity for the protein. Mutant protein in homozygote B cells was expressed at slightly higher levels than endogenous protein but it lacks any detectable kinase activity. (J:146161)