stat1 gene expression assayed by RT-PCR. A. Calibration series. A gradient of RNA dilutions and 21 amplification cycles with stat1b primers verified a semi-quantitative analysis of target mRNAs. B. Detection of stat1a and stat1b expression using RT-PCR at various stages of development using beta actin expression as loading control. RNA samples were extracted from zebrafish embryos at the indicated ages. Abbreviations: –, negative control with no RNA added; u, unfertilized eggs; 0.5, 2, etc., ages at hours post fertilization (hpf).