The claustrum is a subcortical nucleus that exhibits dense connectivity across the neocortex. Considerable recent progress has been made in establishing its genetic and anatomical characteristics, however a core, contentious issue that regularly presents in the literature pertains to the rostral extent of its anatomical boundary. The present study addressed this issue in the rat brain. Using a combination of immunohistochemistry and neuronal tract tracing, we have examined the expression profiles of several genes that have previously been identified as exhibiting a differential expression profile in the claustrum relative to the surrounding cortex. The expression profiles of parvalbumin, crystallin mu (Crym), and guanine nucleotide binding protein (G protein), gamma 2 (Gng2) were assessed immunohistochemically alongside, or in combination with cortical anterograde, or retrograde tracer injections. Retrograde neuronal tracer injections into various thalamic nuclei were used to further establish the rostral border of the claustrum.