| Property | Summary |
|---|---|
| Gene / protein name | **C. elegans:** **mksr-1** (MKSR-1), UniProt Q21191, annotated as **B9 domain-containing protein 1**; **human ortholog:** **B9D1** (also called MKS9 in some literature) (pqac-00000001, pqac-00000005, pqac-00000012) |
| Protein family / domain | Member of the **B9 domain protein family**; contains a **B9-C2 / C2_B9-type domain**, a specialized C2-domain family associated with ciliary proteins and inferred membrane/lipid interaction functions (pqac-00000009, pqac-00000011, pqac-00000012) |
| Subcellular localization | Localizes specifically to the **ciliary transition zone (TZ)** at the base of **sensory cilia** in *C. elegans* amphid and phasmid neurons; mammalian B9D1 is also a TZ protein (pqac-00000001, pqac-00000005, pqac-00000007) |
| Molecular complex | Core component of the **MKS/B9 transition-zone complex**; in vertebrates the B9 subcomplex is organized as **MKS1–B9D2–B9D1**, with B9D2 bridging MKS1 and B9D1 (pqac-00000012, pqac-00000013, pqac-00000014) |
| Functional role | Functions in **ciliary gate / diffusion barrier** activity at the TZ, helping maintain selective ciliary membrane protein composition and compartmentalization rather than acting as a classical enzyme or transporter; broadly supports TZ organization and ciliary signaling homeostasis (pqac-00000015, pqac-00000025, pqac-00000027) |
| Genetic / pathway module | Belongs to the **MKS module** of TZ proteins; specifically behaves as a **core MKS module protein** in the hierarchical assembly pathway downstream of **MKS-5/RPGRIP1L** and **CEP-290**, and is genetically separable from the **NPHP module** while functionally redundant with it during ciliogenesis/barrier formation (pqac-00000017, pqac-00000018, pqac-00000020, pqac-00000022) |
| Key interacting partners | Strongly linked with **MKSR-2/B9D2** and **MKS-1** through interdependent localization in worms; mammalian interaction mapping supports a linear **MKS1–B9D2–B9D1** arrangement. As a core MKS protein, its proper TZ localization also depends on **CEP-290** and upstream **MKS-5**-dependent assembly. Core/peripheral MKS relationships place it functionally alongside **MKS-2/TMEM216**, **TMEM-231**, **TMEM-17**, **TMEM-67**, **TMEM-218**, and **TMEM-237** (pqac-00000001, pqac-00000013, pqac-00000017, pqac-00000018, pqac-00000020) |
| Single-mutant phenotype in *C. elegans* | **mksr-1 single mutants** are generally **non-Dyf** (no obvious dye-filling defect) and do not show major defects in gross ciliogenesis, intraflagellar transport, or chemosensation, indicating MKSR-1 is not absolutely required for basal cilium assembly on its own (pqac-00000003, pqac-00000004, pqac-00000036, pqac-00000040) |
| Double-mutant / synthetic phenotype | Stronger phenotypes emerge in combination with other TZ genes. **MKS/NPHP double mutants** reveal redundant function in ciliogenesis and membrane anchoring, with synthetic defects including **dye-filling defects**, **loss of TZ ultrastructure/Y-links**, **axoneme malformations**, **cilium shortening**, and **TZ displacement**. Genetic interactions are especially evident with **nphp-1**, **nphp-4**, and sensillum-specific interactions with **nphp-2** (pqac-00000008, pqac-00000017, pqac-00000025, pqac-00000036, pqac-00000037, pqac-00000038) |
| Assembly-dependence / localization dependencies | MKSR-1 localization is **interdependent** with other core MKS proteins and is disrupted when **MKSR-2**, **MKS-1**, or **CEP-290** are absent; **MKS-5** acts further upstream as a foundational TZ assembly factor (pqac-00000001, pqac-00000017, pqac-00000018, pqac-00000020, pqac-00000022) |
| Human disease associations of ortholog | **B9D1** is associated with **Meckel syndrome / Meckel syndrome type 9** and **Joubert syndrome / Joubert syndrome 27** in human disease databases and primary literature (pqac-00000000) |


*Table: This table summarizes the main molecular, cellular, genetic, and disease-related properties of C. elegans MKSR-1 and its human ortholog B9D1. It is useful as a compact reference linking transition-zone localization, MKS-module function, mutant phenotypes, and ciliopathy associations.*