A splice acceptor sequence, a 3XSTOP transcriptional/translational termination sequence and a frt site-flanked neomycin resistance cassette (neor), the entire sequence flanked by loxP sites, was inserted into the 5' untranslated region (5'-UTR) 1,046 base pairs downstream of the transcription start site. A single point mutation was introduced into exon 2 that replaces a nucleotide triplet encoding glutamine at amino acid position 61 with an arginine codon (Q61R); this represents the predominant NRAS mutation observed in human melanomas. (J:220627)