A 15.0-kilobase (kb) Eco RI DNA fragment from normal mouse Balb/c genomic DNA that contains sequences (sarc) homologous to the acquired cell sequences (src) of Moloney sarcoma virus (MSV) has been cloned in phage lambda. The sarc region (1.2 to 1.3 kb) of the 15.0-kb cell fragment is indistinguishable from the src region of two isolates of MSV as judged by heteroduplex and restriction endonuclease analyses. The cellular sequences flanking sarc show no homology to other MSV sequences. Whereas cloned subgenomic portions of MSV that contain src transformed NIH-3T3 cells in vitro, the cloned sarc fragment is inactive.