DHCR7 and Smith-Lemli-Opitz syndrome: We developed a gas chromatography-mass spectrometry (GC-MS) protocol to quantify the absolute levels of cholesterol and 7DHC in serum-starved fibroblast samples (Fig. 1B) and demonstrated a dramatic increase in the 7DHC/cholesterol ratio in homozygous transgenic Dhcr7 MEFs (henceforth denoted Dhcr7ΔEx8/ΔEx8), as seen in SLOS patients (Fig. 1C).