SH-Sy5y cells were stimulated with vehicle (Control), 50 nM insulin, or 25 nM IGF-1 for 24 hours, then analyzed by real time quantitative RT-PCR to measure mRNA levels of insulin receptor substrate (IRS) subtypes 1, 2, and 4 using the primer pairs listed in Table 2. The results were normalized to 18S ribosomal RNA measured in the same samples. The values listed represent mean ± S.D. of results obtained from 6 replicate cultures. *P < 0.001 relative to IRS-2 and IRS-4 measured in the same cultures. ++ P < 0.001; + P < 0.05 relative to control.