Outline of the strategy used to generate and express hβc mutants. Schematic illustration of hβc showing the signal sequence (shading), the two cytokine receptor modules (CRMs72) containing the conserved cysteine residues (thin vertical lines) and the characteristic WSXWS motifs (thick vertical lines; see Bazan73 for a description of these elements), and the transmembrane and cytoplasmic domains. Also included is a schematic diagram of the pRUFNeo retroviral expression vector containing the hβc cDNA. The positions in the hβc cDNA of theBamHI, Xho I, Bgl II, and Sal I restriction sites that define the regions subjected to random mutagenesis are shown underneath. The arrows above the cDNA represent the primers used for PCR amplification/mutagenesis of the N-terminal and C-terminal hβc fragments; they lie just outside the restriction sites defining the mutagenized regions (see Materials and Methods).