Peptide aptamers are the newest in the class of "genetic" agents that aid in the 
analysis of cellular processes. Peptide aptamers interact with and can 
inactivate gene products, but don't mutate the DNA that encodes them. Reverse 
analysis with peptide aptamers involves isolating aptamers that interact with a 
specific protein and monitoring the resulting aptamer-induced phenotype. 
Conversely, forward analysis with peptide aptamers involves expressing 
combinatorial libraries of aptamers within an organism and screening for 
aptamer-induced variations in their phenotypes. The two-hybrid system is used in 
both processes to help identify the protein interactions, and variations from 
the basic procedure as described in UNIT are presented here.