CD44 is a cell-surface glycoprotein postulated to play a role in a variety of biological processes, including lymphocyte homing and tumor-cell metastasis. Several isoforms of CD44 have been identified in human cells, and the genesis of some of these isoforms has been attributed to alternative splicing. Understanding of mechanisms regulating CD44 alternative splicing may provide insights into diverse processes, including tumor-cell metastasis and lymphocyte homing (1). The deduced protein sequence of 699 amino acids has a novel 339 amino acid domain inserted into the proximal extracellular domain of the standard, leukocyte form of CD44. The additional domain adds a number of potential N- and O-linked glycosylation sites and two proteolysis sites to this form of CD44 (2). CD44 is defined as a structurally diverse, but immunologically related, set of intrinsic membrane macromolecules, and suggests that these structurally varied forms might be expected to manifest multiple functions. (3)