Component of the spindle-assembly checkpoint that prevents the onset of anaphase until all chromosomes are properly aligned at the metaphase plate (By similarity). 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 (By similarity). Homodimer (By similarity). Dimerizes via its N- and C- terminal regions (By similarity). Heterodimerizes with MAD2L1 in order to form a tetrameric MAD1L1-MAD2L1 core complex (By similarity). Interacts with the closed conformation form of MAD2L1 (C-MAD2) and open conformation form of MAD2L1 (O-MAD2) (By similarity). It is unclear whether MAD1L1 dimerization promotes the conversion of closed to open conformation of MAD2L1 (By similarity). 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 (By similarity). Perturbation of the original MAD1L1-MAD2L1 structure by the spindle checkpoint may decrease MAD2L1 affinity for MAD1L1 (By similarity). 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 (By similarity). Also able to interact with the BUB1/BUB3 complex (By similarity). Interacts with NEK2 (By similarity). Interacts with TTK (By similarity). Interacts with TPR; the interactions occurs in a microtubule-independent manner (By similarity). Interacts with IK (By similarity). Interacts with the viral Tax protein (By similarity). Interacts with PRAP1 (By similarity). Detected at the nucleus envelope during interphase. 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. Detected at kinetochores during prometaphase. Colocalizes with NEK2 at the kinetochore. Colocalizes with IK at spindle poles during metaphase and anaphase. Phosphorylated; by BUB1. Become hyperphosphorylated in late S through M phases or after mitotic spindle damage. Belongs to the MAD1 family.