Cellular communication is an essential aspect of animal development, and is often mediated by proteins exhibiting modular structures. Two of the most common modules found in signaling proteins are the leucine-rich repeat (LRR) and immunoglobulin-like (Ig) domain. Individually, LRRs and Ig domains often dictate protein-protein interactions, but little is known about the functional relevance of proteins which contain both modules, termed LIG proteins. One apparent trend is that LIG proteins act as mediators and modulators of cellular communication. The Drosophila genome encodes nine LIG proteins, of which six appear to be evolutionarily related and represent the Kekkon (Kek) family. One member of this family, Kek1, acts as a direct inhibitor of the Epidermal Growth Factor Receptor (EGFR), but the five remaining Kek family members remain uncharacterized. I have generated mutations in a second Kek family gene, kek5, and have shown that kek5 mutants display defects in vein patterning and epithelial adhesion in the wing. Through genetic and molecular analyses, I have determined that Kek5 acts as a modulator of Bone Morphogenetic Protein (BMP) signaling during wing development. Unlike Kek1, Kek5 does not act as a direct inhibitor of receptor activity, but appears to regulate the extracellular distribution of BMP ligands. Structure/function analysis of Kek5 supports this idea by demonstrating that the extracellular LRRs of Kek5 are required for its activity. Further, I have shown that kek5 mutants display strong genetic interaction with Integrins, transmembrane proteins that mediate adhesion in the wing. Finally, phylogenetic analysis of the Kek family reveals that these proteins are evolutionarily conserved throughout the arthropod lineage, suggesting that they may represent ancient modulators of cellular communication.