ASB9 (ankyrin repeat and SOCS box containing 9) is an X-linked E3 ubiquitin ligase that functions as a substrate-recognition subunit of ECS-type Cullin RING ligase complexes 1. The protein contains ankyrin repeat domains for substrate binding and a SOCS box domain that facilitates assembly with Elongin B/C, Cullin-2, and RBX1 to form the ECSASB9 complex 2. ASB9 catalyzes K48-linked polyubiquitination of target proteins, directing them for proteasomal degradation 2. Known substrates include TUBB4A (tubulin beta 4 A) in testis 2 and HIF1AN in spermatogonial stem cells 3. In spermatogenesis, ASB9 is highly expressed in nonproliferating spermatogonial stem cells, where it suppresses proliferation and promotes apoptosis through HIF1AN degradation 3. ASB9 also regulates ovarian granulosa cell function by modulating MAPK signaling through interactions with PAR1 and TAOK1 4. Clinically, ASB9 dysfunction is implicated in male infertility; genetic variants in ASB9 are associated with idiopathic infertility and oligoasthenoteratozoospermia 2, and ASB9 dysregulation correlates with non-obstructive azoospermia 3. Additionally, X-chromosome X near ASB9 show genome-wide association with autism spectrum disorder 5.