This family includes the HABP4 family of hyaluronan-binding proteins, and the PAI-1 mRNA-binding protein, PAI-RBP1. HABP4 has been observed to bind hyaluronan (a glucosaminoglycan), but it is not known whether this is its primary role in vivo. It has also been observed to bind RNA, but with a lower affinity than that for hyaluronan. PAI-1 mRNA-binding protein specifically binds the mRNA of type-1 plasminogen activator inhibitor (PAI-1), and is thought to be involved in regulation of mRNA stability. However, in both cases, the sequence motifs predicted to be important for ligand binding are not conserved throughout the family, so it is not known whether members of this family share a common function.