Table I. Potential mechanisms for the generation and function of cell surface, matrix-associated, and fluid phase CD44.

Form of CD44	Potential mechanisms of generation	Known and proposed functions
Transmembrane	Synthesized as type-I integral transmembrane protein	1. Cell–cell adhesion 2. Cell–matrix adhesion 3. Signal transduction 4. Pericellular matrix assembly 5. Hyaluronan metabolism
Matrix-associated ectodomain	1. Shedding of transmembrane form 2. Secretion of alternatively spliced form	1. Pericellular matrix assembly 2. Integral component of ECM 3. Agonist/antagonist of transmembrane form
Fluid phase	1. Shedding of transmembrane form 2. Secretion of alternatively spliced form 3. Enzymatic liberation from matrix	Agonist/antagonist of transmembrane- and matrix- associated forms
Intracellular cytoplasmic domain	Presenilin-dependent cleavage of truncated transmembrane form	Regulation of transcription after nuclear translocation