Our investigation of the effect of the DNA sequence (GA)n.(CT)n on DNA replication has been continued. Synthetic DNA of this sequence, 80 bp in length, was cloned in E. coli. When it was attempted to insert this sequence into SV40, all of the SV40 isolates grew slowly and had suffered a large deletion of the (GA)n.(CT)n sequence, whereas random DNA of the same length gave SV40 variants of the expected size and grew normally. We are currently attempting to use the new blotting procedure of Brewster and Fangman to demonstrate the inhibitory effect of this sequence on DNA replication. We have continued our investigation of the "O-family" sequence. We have demonstrated the presence of a protein that site- specifically interacts with a portion of the sequence. In collaboration with Dr. Bruce Howard, we have demonstrated that the O-family sequence on transfection into animal cells inhibits DNA synthesis. A small deletion that obliterates the site specific protein interaction, also diminishes the inhibitory effect of the O-family sequence on DNA replication. We continue to attempt to demonstrate transposon-like activity for the O-family sequences.