The actin filament system is essential for many cellular functions, including shape, motility, cytokinesis, intracellular trafficking, and tissue organization. Tropomyosins (Tms) are rod-like components of most actin filaments that differentially affect their stability and flexibility. The Tm gene family consists of four genes, ␣Tm, ␤Tm, ␥Tm (Tm5 NM, where "NM" indicates "nonmuscle"), and ␦Tm (Tm4). Multiple isoforms of the Tm family are generated by alternative splicing of three of these genes, and their expression is highly regulated. Extensive spatial and temporal sorting of Tm isoforms into different cellular compartments has been shown to occur in several cell types. We have addressed the function of the low-molecular-weight Tms encoded by the ␥Tm gene by eliminating the corresponding amino-terminal coding sequences from this gene. Heterozygous mice were generated, and subsequent intercrossing of the F 1 pups did not result in any viable homozygous knockouts. Genotype analysis of day 2.5 morulae also failed to detect any homozygous knockouts. We have failed in our attempts to delete the second allele and generate in vitro double-knockout cells, although 51 clones displayed homologous recombination back into the originally targeted locus. We therefore conclude that lowmolecular-weight products from the ␥Tm gene are essential for both embryonic development and cell survival.

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A genomic clone and cDNA fragment encoding a portion of a humoral recognition molecule from the aglish were isolated and sequenced. The serum protein has previously been described as having structural features that are immunoglobulin-like. Amino acid sequence obtained from the 77-kDa Hi heavy chain facilitated the isolation of a genomic clone containing at least two coding regions. The hagfish is an agnathan, a modem representative of the earliest evolved group of vertebrates, the ostracoderms or jawless fishes, which arose prior to the ancient placoderms (3). As the "most primitive" living vertebrate, the hagfish occupies a key position in the scheme of evolution of metazoan immunity, linking the vertebrates with their invertebrate ancestors, the protochordates (4). We have reported (5) the isolation and partial characterization of a protein with carbohydrate-binding specificity from the serum of Pacific hagfish (Eptatretus stouti) immunized with a streptococcal whole-cell vaccine. The molecule was found to exhibit a subunit and polypeptide chain structure similar to that of Ig (6, 7). However, the hagfish protein was unique in that it appeared to include a heterodimer of structurally distinct heavy chains (H1, =77 kDa; H2, =70 kDa). Light ITo whom reprint requests should be addressed. 7910The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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