THE JOHNS HOPKINS UNIVERSITY 
SCHOOL OF MEDICINE 
DEPARTMENT OF MICRORIOLOGY 
72.5 N. WOLFE STREET 
BALTIMORE, MARYLAND 21205 
December 12, 1979 
Dr. Donald Frederickson 
Director 
National Institutes of Health 
Bethesda, MD 20205 
Dear Dr. Frederickson: 
I am writing to support your recent decision to reduce to the PI 
level containment requirements for recombinant DNA experiments involving 
£. col i K12 not harboring conjugation-proficient plasmids or generalized 
transducing phages when lambda or lambdoid phages are used as vectors. 
In my opinion, PI containment provides adequate safeguards for these 
experiments, as it does for known enteric pathogens. 
Daniel Nathans 
Professor and Director 
Department of Microbiology 
DN/lc 
[ 461 ] 
