Soggy-1, also known as DkkL1 (Dickkopf-like 1), is a secreted protein that is related to the Dickkopf family of Wnt antagonists. Dkk-1, -2, -3, and -4 each have two cysteine-rich domains separated by a linker. Soggy-1 does not contain cysteine-rich domains but bears some sequence homology with the N-terminal regions of Dkk proteins (1, 2). The mouse Soggy-1 cDNA encodes a 230 amino acid (aa) precursor that includes a 20 aa signal sequence (2). Mouse Soggy-1 shares 65% and 90% aa sequence identity with human and rat Soggy-1, respectively, and approximately 15% aa sequence identity with mouse Dkk-1, -2, -3, and -4. Mouse Soggy-1 is expressed at various sites in the embryo but in the adult is primarily found in the testes (3 - 5). Soggy-1 transcription is regulated by the spermatocyte specific factor, RFX2 (6). The regulatory elements for Soggy-1 lie very close to those of TEAD-2, a transcription factor that is expressed very early in development (4). Soggy-1 and TEAD-2 are co-expressed in preimplantation embryos and embryonic stem cells, but differentiated cells express only one or the other (4, 5). During development, Soggy-1 is first detectable at the onset of sexual differentiation (4). Soggy-1 is localized to the acrosome in developing spermatocytes and mature spermatozoa (3, 4). Soggy-1, as expressed in developing mouse spermatocytes, is a 34 kDa N-glycosylated protein. This glycosylation is not present on Soggy-1 in mature spermatozoa, although the apparent molecular weight suggests the presence of some post-translational modification (3). Two shorter forms of Soggy-1 have been described that result from the use of internal methionine residues for initiation (4).