Table 1.

Phenotypic categorization of SI mutants

Phenotype of SI mutants	Transport and function
I	Blocked in the ER, enzymatically inactive
II	Blocked in the ER, ER-Golgi intermediate compartment, and cis-Golgi, inactive
III	Normal intracellular protein transport but absent activities
IV	Sorting defects, enzymatically active
V	Intracellular proteolytic cleavage and degradation of sucrase, normal transport and partial activity
VI	Intracellular proteolytic cleavage and secretion of SI, complete activity
VII	Altered trafficking and increased turnover rate, partially active