The Asbs are a family of ankyrin repeat proteins that, along with four other protein families, contain a C-terminal SOCS box motif, which was first identified in the suppressor of cytokine signaling (SOCS) proteins. While it is clear that the SOCS proteins are involved in the negative regulation of cytokine signaling, the biological roles of the other SOCS box-containing families are unknown. We have investigated Asb-1 function by generating mice that lack this protein, as well as mice that overexpress full-length or truncated Asb-1 in a wide range of tissues. Although Asb-1 is expressed in multiple organs, including the hematopoietic compartment in wild-type mice, Asb-1-/- mice develop normally and exhibit no anomalies of mature blood cells or their progenitors. While most organs in these mice appear normal, the testes of Asb-1-/- mice display a diminution of spermatogenesis with less complete filling of seminiferous tubules. In contrast, the widespread overexpression of Asb-1 in the mouse has no apparent deleterious effects.