Primer-BLAST was developed at NCBI to help users make primers that are specific
 to intended PCR target.  It uses Primer3 to design PCR primers and then uses 
BLAST and global alignment algorithm to screen primers against user-selected database
in order to avoid primer pairs (all combinations including forward-reverse primer pair,
 forward-forward as well as reverse-reverse pairs) that can cause non-specific amplifications. 
To cite Primer-BLAST or look for more details, please consult our publication:

Ye J, Coulouris G, Zaretskaya I, Cutcutache I, Rozen S, Madden T (2012).
Primer-BLAST: A tool to design target-specific primers for polymerase chain reaction.
BMC Bioinformatics. 13:134.

Use of Primer3 itself is subject to the terms and conditions stated on Primer3 web site.