XB51 (derived from X11-like binding protein of clone number 51) was isolated by yeast two-hybrid cDNA screening using the N-terminal domain of X11L (X11-like protein) as a bait. X11L is a neuron-specific adaptor protein that is known to down-regulate APP (b-amyloid precursor protein) metabolism by associating with the cytoplasmic domain of APP, but the detailed mechanisms are still unknown. Thus the X11L-associated protein XB51 is believed to regulate APP metabolism by modifying X11L function through its interaction with X11L. Here we report that the hXB51 (human XB51) gene can yield two transcripts, one with exon 9 spliced out (resulting in the hXB51b isoform) and the other containing exon 9 (yielding the hXB51a isoform). hXB51a binds to X11L to form a tripartite complex composed of hXB51a, X11L and APP. Complex-formation results in blocking X11L's suppression of Ab (b-amyloid) generation from APP. hXB51b associates with X11L and inhibits its interaction with APP. However, hXB51b suppresses Ab generation and secretion in an X11L-independent manner. Thus the hXB51 isoforms regulate Ab generation differently, either enhancing it by modifying the association of X11L with APP or suppressing it in an X11L-independent manner. These observations advance our understanding of the molecular mechanisms regulating intracellular Ab production and the pathogenesis of Alzheimer's disease.