342 neurons in tetrapods, heterotypic coupling may seem at ﬁrst glance to preclude rectiﬁcation. However, functional asymmetry between apposed hemichannels could perhaps be achieved by differences in posttranslational modiﬁca- tions between these two channels. Supporting this possi- bility, Cx36-containing junctions have been shown to coexist at different degrees of phosphorylation in the retina (Kothmann et al. 2009). If found in the mammalian CNS, electrical rectiﬁcation and modulation of rectiﬁcation would open up new pathways for synaptic communication that may be of particular relevance in the human CNS and in human neurological disease. Future Directions As powerful as FRIL has proven to be, there are two limita- tions that make it and DR-FRIL less inviting for the novice. First, freeze-fracture and FRIL require complex, costly equipment that is at the cutting edge of high-vacuum, cryo- preservation and metal evaporation technologies. Second, with more than a dozen discrete steps, all of which must be performed ﬂawlessly to obtain the matching complementary replicas, DR-FRIL is particularly demanding of its techno- logical practitioners. Third, freeze-fracture electron micros- copy requires considerable time to gain expertise in obtaining and interpreting FRIL images. The training period for FRIL is several years because the researcher must learn conventional electron microscopy as well as interpretation of tissue ultra- structure in freeze-fracture replicas before learning FRIL methods. In these days of automated molecular biology, few students are willing to make this long-term commitment. 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