Mapping truncations to the human β-mannosidase protein. The extent of truncated mutants caused by nonsense, insertion and deletion mutations are shown as red, orange and green arrows, representing severe, moderate and mild phenotypes, respectively. Truncations in goat (broken arrow) and cow are shown below protein chain (blue rectangle). The purple and yellow arrow-heads represent the locations of the binding site residues and the catalytic nucleophiles, respectively. The corresponding single letter amino acid codes and ther residue numbers are given below the lower arrow-heads.