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porphin

[pôr′fĭn]
n.
  1. The unsubstituted heterocyclic ring consisting of four pyrrole rings; it is the basic unit of the porphyrins.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.