At birth and thereafter (P0-P12 analyzed), the expression of a7 GFP was strongly detected in SG afferent fibers where they terminate near or at the base of IHC sensory cells ( Fig. 5H and I) . This basic pattern of a7 GFP expression was reinforced during the remaining postnatal period as fibers continue to form a dense plexus that appears to surround the base of the IHCs. The other efferent fibers not detected by a7 GFP continue to be trimmed and also associate with their final targets (Merchan-Perez and Liberman 1996; Simmons et al. 1996; Hafidi 1998; Huang et al. 2007 ). The outcome of this remodeling was evident by P12 when the SG1 afferent terminals surrounding the IHC were distinguished by strong a7 GFP staining of the terminal clusters ( Fig. 5I and inset) . This was approximately the same time hearing onset occurs in mice (~P10; Kros et al. 1998 ). Processes originating from SG cells identified by peripherin expression that were not colabeled with a7 GFP form distinct efferent terminals on or very near OHCs cells and on the terminals that end on the IHC afferent terminals identified by a7 GFP labeling ( Fig. 5I ; Huang et al. 2007 ). While not entirely evident from the images shown, not all SG cells at P12 expressed a7 GFP , suggesting this could identify a functionally distinct subpopulation ( Fig. 5I ; Happe and Morley 1998) . Again, no a7 GFP labeling of olivocochlear efferents was detected. A diagram summarizing these findings is shown in Fig. 5J .