Component of the spindle-assembly checkpoint that prevents the onset of anaphase until all chromosomes are properly aligned at the metaphase plate (PubMed:10049595, PubMed:20133940, PubMed:29162720). Forms a heterotetrameric complex with the closed conformation form of MAD2L1 (C-MAD2) at unattached kinetochores during prometaphase, recruits an open conformation of MAD2L1 (O-MAD2) and promotes the conversion of O-MAD2 to C-MAD2, which ensures mitotic checkpoint signaling (PubMed:29162720). Sequesters MAD2L1 in the cytoplasm preventing its function as an activator of the mitotic spindle assembly checkpoint (SAC) resulting in SAC impairment and chromosomal instability in hepatocellular carcinomas. Homodimer (PubMed:9546394, PubMed:29162720). Dimerizes via its N- and C- terminal regions (PubMed:29162720). Heterodimerizes with MAD2L1 in order to form a tetrameric MAD1L1-MAD2L1 core complex (PubMed:22351768, PubMed:9546394, PubMed:18981471, PubMed:12006501). Interacts with the closed conformation form of MAD2L1 (C-MAD2) and open conformation form of MAD2L1 (O-MAD2) (PubMed:29162720). It is unclear whether MAD1L1 dimerization promotes the conversion of closed to open conformation of MAD2L1 (PubMed:29162720). Formation of a heterotetrameric core complex containing two molecules each of MAD1L1 and of MAD2L1 promotes binding of another molecule of MAD2L1 to each MAD2L1, resulting in a heterohexamer (PubMed:12006501). Perturbation of the original MAD1L1-MAD2L1 structure by the spindle checkpoint may decrease MAD2L1 affinity for MAD1L1 (PubMed:12006501). CDC20 can compete with MAD1L1 for MAD2L1 binding, until the attachment and/or tension dampen the checkpoint signal, preventing further release of MAD2L1 on to CDC20 (PubMed:12006501). Also able to interact with the BUB1/BUB3 complex (PubMed:10198256). Interacts with NEK2 (PubMed:14978040). Interacts with TTK (PubMed:29162720). Interacts with TPR; the interactions occurs in a microtubule-independent manner (PubMed:18981471, PubMed:19273613, PubMed:20133940). Interacts with IK (PubMed:22351768). Interacts with the viral Tax protein (PubMed:9546394). Interacts with PRAP1 (PubMed:24374861). Interacts with MAD2L1; this interaction leads to the cytoplasmic sequestration of MAD2L1 (PubMed:19010891). Interacts with PRAP1 (PubMed:24374861). Co-localizes with TPR at the nucleus envelope during interphase and throughout the cell cycle (PubMed:22351768, PubMed:18981471). From the beginning to the end of mitosis, it is seen to move from a diffusely nuclear distribution to the centrosome, to the spindle midzone and finally to the midbody (PubMed:9546394). Localizes to kinetochores during prometaphase (PubMed:22351768, PubMed:29162720). Does not localize to kinetochores during metaphase (PubMed:29162720). Colocalizes with NEK2 at the kinetochore (PubMed:14978040). Colocalizes with IK at spindle poles during metaphase and anaphase (PubMed:22351768). Expressed in hepatocellular carcinomas and hepatoma cell lines (at protein level). Expressed in hepatocellular carcinomas and hepatoma cell lines (at protein level). Increased by p53/TP53. Phosphorylated; by BUB1 (PubMed:10198256). Become hyperphosphorylated in late S through M phases or after mitotic spindle damage (PubMed:9546394). Phosphorylated; by TTK (PubMed:29162720). Defects in MAD1L1 are involved in the development and/or progression of various types of cancer. Belongs to the MAD1 family.