The entire MAD1L1 gene is approximately 417.1 Kb (start: 1815792 and end: 2232971 bp; orientation: Minus strand). The MAD1L1 gene encodes for 6 transcript variants. The transcript variant 1 (start: 1815792 and end: 2232948 bp; orientation: Minus strand; 19 exons; mRNA: 2754 bp) is the longest transcript variant. Transcript variant 2 (mRNA: 2717 bp), transcript variant 3 (mRNA: 2714 bp) and transcript variant 4 (mRNA: 2538 bp) differs in the 5 UTR compared to transcript variant 1. The transcript variants 1-4 encode the protein isoform a (718 aa protein). The transcript variant 5 (mRNA: 2376 bp; 17 exons) lacks some exons in the 5 UTR and displays an alternate exon in the 5 coding region, what results in an alternative start codon, frameshift, and a shorter and distinct protein (isoform b; 626 aa). The transcript variant 6 is the shortest transcript variant (start: 1815792 and end: 1940550 bp; 4 exons; mRNA: 1318 bp), it displays an alternative 5-terminal exon, uses an in-frame start codon and encodes the protein isoform c (174 aa).

MAD1L1 is a 718-residue coiled-coil protein that interacts with MAD2L1, and is composite of a MAD1L1 oligomerization domain, MAD2L1-binding domain and RLK motif (Arg-Leu-Lys; essential for BUB1 and BUB3 interactions) (Canman et al., 2002; Sironi et al., 2002) (Figure 1). MAD1L1-MAD2L1 complexes form a tetramer-like structure due to the parallel intermolecular coiled-coil between α 1 helices from different MAD1L1 molecules (Nasmyth, 2005; Sironi et al., 2002).

Ubiquitous.

During mitosis, MAD1L1 is localized predominantly at unattached kinetochore. During interphase, MAD1L1 is localized in perinuclear region, nuclear pore complex, centrosome and Golgi apparatus (Campbell et al., 2001; Rodriguez-Bravo et al., 2014; Wan et al., 2014).

MAD1L1 belongs to the assembly control of the mitotic spindle, which acts as a component capable of blocking the onset of the anaphase if the chromosomes are not correctly aligned on the metaphase plate (Hardwick and Murray, 1995). The MAD1L1 protein has approximately 83 kDa and interacts with other proteins, such as MAD2L1, to perform its functions (Campbell et al., 2001; Wright et al., 2017). The complex MAD1L1/MAD2L1 binds to improperly attached kinetochrore, induces MAD1L1 phosphorylation by MPS1, and acts as an anchor for the formation of other protein interactions, including MAD2L1-CDC20 complex. The molecular events upon MAD1L1 and unattached kinetochores interaction lead to the formation of mitotic checkpoint complex (MCC), inhibition of anaphase-promoting complex/cyclosome (APC/C), and cell cycle arrest (Reviewed by Schuyler et al., 2012 and London and Biggins, 2014) (Figure 2).
Rodriguez-Bravo and colleagues (Rodriguez-Bravo et al., 2014) reported that the MAD1L1-MAD2L1 complex anchored to the nuclear pore complexes emits an inhibitory signal that limits the speed of the mitosis, facilitating corrections of possible errors, which would avoid a cell cycle arrest. Alterations in MAD1L1 are associated with chromosomal instability and aneuploidy (Avram et al., 2014; Tsukasaki et al., 2001).
Wan and colleagues (Wan et al., 2014) reported an elegant mechanistic study that described the non-related spindle assembly checkpoint and MAD2L1 independent functions for MADlL1. During interphase, MAD1L1 is localized in the Golgi apparatus, and participates in integrin secretion, adhesion, mobility, cell migration and PTK2 (FAK) signaling pathway (Wan et al., 2014).

MAD1L1 has a high homology among different species (Table 1).
Table 1. Comparative identity of human MAD1L1 with other species

% Identity for: Homo sapiens MAD1L1	Symbol	Protein	DNA
vs. P. troglodytes	MAD1L1	99.4	99.4
vs. M. mulatta	MAD1L1	98.4	96.7
vs. C. lupus	MAD1L1	84.0	84.3
vs. B. taurus	MAD1L1	84.7	85.4
vs. M. musculus	Mad1l1	81.4	81.4
vs. R. norvegicus	Mad1l1	83.4	82.3
vs. G. gallus	MAD1L1	70.0	73.2
vs. X. tropicalis	mad1l1	63.0	65.0
vs. D. rerio	mad1l1	54.1	60.6
vs. D. melanogaster	Mad1	28.9	45.0
vs. A. gambiae	AgaP_AGAP006632	28.8	47.7

(Source: http://www.ncbi.nlm.nih.gov/homologene)

Recurrent mutations in the MAD1L1 gene are rare, and 153 missense substitution, 69 synonymous substitution, 8 nonsense substitution, 1 frameshift insertion and 8 frameshift deletion mutations are reported in COSMIC (Catalogue of Somatic Mutations in Cancer; http://cancer.sanger.ac.uk/cancergenome/projects/cosmic). Similar findings were reported in cBioPortal (http://www.cbioportal.org), which includes 41824 cancer samples: somatic mutation frequency in MAD1L1 was 0.5% (195 mutations, being 166 missense and 29 truncating mutations). Analyzing together, mutation, amplification, deep deletion and multiple alterations, the total of cancer samples with any type of alteration was 570 (1.4%).