Involved in chromosome cohesion during cell cycle and in DNA repair. Central component of cohesin complex. The cohesin complex is required for the cohesion of sister chromatids after DNA replication. The cohesin complex apparently forms a large proteinaceous ring within which sister chromatids can be trapped. At anaphase, the complex is cleaved and dissociates from chromatin, allowing sister chromatids to segregate. The cohesin complex may also play a role in spindle pole assembly during mitosis. Involved in DNA repair via its interaction with BRCA1 and its related phosphorylation by ATM, or via its phosphorylation by ATR. Works as a downstream effector both in the ATM/NBS1 branch and in the ATR/MSH2 branch of S-phase checkpoint. Forms a heterodimer with SMC3 in cohesin complexes (PubMed:22628566). Cohesin complexes are composed of the SMC1 (SMC1A or SMC1B) and SMC3 heterodimer attached via their SMC hinge domain, RAD21 which link them, and one STAG protein (STAG1, STAG2 or STAG3), which interacts with RAD21 (PubMed:11076961). In germ cell cohesin complexes, SMC1A is mutually exclusive with SMC1B (By similarity). Interacts with BRCA1 (PubMed:11877377). Found in a complex with CDCA5, SMC3 and RAD21, PDS5A/SCC-112 and PDS5B/APRIN (PubMed:15837422). Interacts with NDC80 (PubMed:9295362, PubMed:10409732,). Interacts with BRAT1 (PubMed:22977523). Found in a complex containing POLE and SMC3. Interacts with RPGR, STAG3 and SYCP2 (By similarity). Found in a cohesin complex with SMC3, STAG1 and RAD21 (PubMed:22628566). The SMC1A-SMC3 heterodimer interacts with the NIPBL-MAU2 heterodimer (PubMed:22628566). Associates with chromatin. Before prophase it is scattered along chromosome arms. During prophase, most of cohesin complexes dissociate from chromatin probably because of phosphorylation by PLK, except at centromeres, where cohesin complexes remain. At anaphase, the RAD21 subunit of the cohesin complex is cleaved, leading to the dissociation of the complex from chromosomes, allowing chromosome separation. In germ cells, cohesin complex dissociates from chromatin at prophase I, and may be replaced by a meiosis-specific cohesin complex. The phosphorylated form on Ser-957 and Ser-966 associates with chromatin during G1/S/G2 phases but not during M phase, suggesting that phosphorylation does not regulate cohesin function. Integral component of the functional centromere-kinetochore complex at the kinetochore region during mitosis. The flexible SMC hinge domain, which separates the large intramolecular coiled coil regions, allows the heterotypic interaction with the corresponding domain of SMC3, forming a V-shaped heterodimer. The two heads of the heterodimer are then connected by different ends of the cleavable RAD21 protein, forming a ring structure (By similarity). Ubiquitinated by the DCX(DCAF15) complex, leading to its degradation. Phosphorylated by ATM upon ionizing radiation in a NBS1-dependent manner. Phosphorylated by ATR upon DNA methylation in a MSH2/MSH6-dependent manner. Phosphorylation of Ser-957 and Ser-966 activates it and is required for S-phase checkpoint activation. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Mutated Cornelia de Lange cell lines display genomic instability and sensitivity to ionizing radiation and interstrand cross-linking agents. Belongs to the SMC family. SMC1 subfamily.