g 13 genes in mammals [1-3]. Both are heterodimeric enzymes consisting of distinct 80 kDa catalytic subunits, encoded by the Capn1 (μ-80 k) and Capn2 (m-80 k) genes, respectively, that associate with a common 28 kDa regulatory subunit encoded by the Capn4 gene. The μ-80 k and m-80 k subunits share 62% amino acid sequence identity, and are very similar in terms of structure, protein chemistry, and in vitro substrate specificity. Despite these similarities, the differential expression patterns of μ- and m-calpain in mammalian tissues suggest they have some isoform specific and distinct functions. The μ and m designations derive from the levels of Ca2+ required in vitro for optimal activation; 10–50 μM Ca2+ for μ-calpain and 0.3–0.35 mM Ca2+ for m-calpain. It is generally assumed that μ- and m-calpain maintain their differential sensitivities to calcium in vivo, although this has not yet been strictly demonstrated. Furthermore, since the cytoplasmic free Ca2+ concentration is typically less than 1 μM, it is also assumed that other in vivo factors must contribute to regulation of these enzymes [3]. Without apriori knowledge of the factors regulating calpain activity or their relevant substrates, elucidation of biological functions for calpains presents a challenge. Research on calpains has linked them with a wide variety of functions including muscle growth, development, degeneration (3), neuronal growth and neurodegeneration [4], cell cycle progression [5,6], signal cascades triggered by integrins and growth factors [7], membrane protrusion [8], remodeling of the cytoskeleton and cell migration [9-15], and regulation of cell death via both necrosis and apoptosis [16-22]. To date, the literature suggests a complex interplay between caspases and calpains [23,24] and impact of calpain on cell death pathway components [25]. The lack of highly specific cell-permeable inhibitors of calpains contributes to the challenge of investigating and defining calpain functions in these processes. Although over-expression of calpastatin, the endogenous protein inhibitor of μ- and m-calpain provides an important approach for these efforts, it will not distinguish isoform specific functions [24,26,27]. Some work has suggested isoform specific roles, such as a role for m-calpain in epidermal growth factor (EGF)-induced cell motility [28,29] and a role for μ-calpain in interferon-inducible protein 9-induced migration of keratinocytes [28]. A cell permeable calpain inhibitor (which likely inhibits other thiol-proteases as well) has been used to select cells lacking μ-calpain which display reduced proliferation rates [30]. Interestingly, m-calpain expression persisted in these cells, suggesting a possible requirement of m-calpain for cell survival [30]. Targeted gene deletion in mice provides a powerful approach to determining the physiological roles of μ- and m-calpain and the opportunity to approach their isoform specific functions. Initial st