The modular structure of insulin-receptor substrates IRS-1 and IRS-2. This schematic view represents the amino acid sequence common to IRS-1 and IRS-2. Each protein contains a pleckstrin homology domain (which binds phosphoinositide lipids), a phosphotyrosine-binding domain, and four sequences that approximate Tyr-X-X-Met (YXXM). The four sequences are phosphorylated by the insulin-receptor tyrosine kinase.