Induction of expression of early response genes by NGF in HMC-1 mast cells. HMC-1 cells were stimulated with NGF, at the concentrations indicated, for 15, 30, or 60 minutes. Total RNA was extracted and Northern blots were hybridized sequentially to mouse cDNAs of c-fos and zif/268 (NGF1-A). Ethidium bromide staining was used to assess loading of RNA.