Retroviral vectors and packaging cell lines. (A) LK; (B) LN-ras2. LTR, M-MLV long terminal repeat; Ψ, packaging region; thick vertical line, viral splice donor site; parallel vertical lines, polylinker; arrows, primers used for PCR. (C and D) Immunohistochemical staining for human N-ras protein. (C) ΨLN-ras215 cells show cytoplasmic staining of N-ras protein. (D) ΨLN-ras2 nonproducers (LN-ras2 transfectant that did not make detectable virus). Mophology of (E), ΨLN-ras215 producers; (F) ΨLK09 producers (original magnification ×250).