OOEP, alias oocyte expressed protein, is a protein coding gene that starts at 73,368,555 nt and ends at 73,369,792nnt from qter and with a length of 1238 bp. The current reference sequence is NC_000006.12 and contain 3 exons. It is proximal to KHDC3L (KH domain containing 3 like, subcortical maternal complex member) gene and to RPL39P3 (ribosomal protein L39 pseudogene 3) gene. Around the genomic locus of OOEP take place different promoter or enhancer transcriptional elements. Two strong elements are closer to the sequence of OOEP gene and are located at +0.3 kb and at -27.3 kb respectively.

OOEP transcript is 689 bp long with a reference sequence reported in GeneBank as NM_001080507. It lacks the 5 UTR, the CDS is extended from 1 to 450 nt and the 3 UTR is extended in the remaining part of the sequence, i.e. from 451 to 689 nt.
Splice variants for OOEP was observed: the main reference variant is OOEP (OOEP-201) and the others are OOEP-202 and OOEP-203 (Figure.1). OOEP-202, 1007 nt long, is formed by a fragment of exon 3, by the entire exon 2, it lacks the first exon and gains a forth distant element. OOEP-203, 964 nt long, lacks exons 3 and 1, maintains the entire exon2 and gains another distant element. All three transcript variants encode a protein.

For OOEP are known some pseudogenes that are classified as processed pseudogenes and are listed in Table 1.

Gene	Gene  name	RefSeq	Locus	Location	Start	End	Lenght (nt)
AL499605.1-201	OOEP pseudogene	ENST00000604628.1	1p21.1	Chrom. 1	106544342	106544744	403
AL355333.1	OOEP pseudogene	ENSG00000270234	10p14	Chrom. 10	8724010	8724288	279

Table.1 OOEP pseudogenes (reworked from https://www.ncbi.nlm.nih.gov/gene/1937)

The canonical sequence for OOEP protein (RefSeq NP_001073976) counts 149 amino acids and has a molecular weight of 17.17 kDa and a theoretical pI of 6.59. Contains a KH-domain, a typical domain of the type I superfamily of RNA binding proteins (Herr et al., 2008), that could mediate RNA transcript regulation during the oogenesis and early embryogenesis stages.
There are known other two isoforms produced by alternative splicing: the isoform OOEP-202 (UniRef, F2Z364) is formed by 94 residues and has a molecular weight of 10.72 kDa, while the isoform OOEP-203 (UniRef, C9J915) counts 67 amino acids and has 7.70 kDa of molecular weight.

OOEP, as the others factors of the subcortical maternal complex (SCMC), is uniquely expressed in mammalian oocytes and in early embryo (Bebbere et al., 2016). However, some authors found mouse OOEP transcripts also in ovary and thymus, although the protein could not be detected. This may suggest that the transcript remains untranslated (Herr et al., 2008) and perhaps plays a regulatory function. The human OOEP mRNA was found in pituitary gland (Herr et al., 2008; Carninci and Hayashizaki, 1999), placenta (https://www.ncbi.nlm.nih.gov/gene) and testis, where it was overexpressed (https://www.gtexportal.org/home/gene/ENSG00000203907; https://genevisible.com/ tissues/HS/UniProt/A6NGQ2). It was also found in traces in ovary, endometrium, prostate, salivary gland, adrenal, appendix, brain, digestive system and related organs (esophagus, stomach, duodenum, small intestine, colon, gall bladder, liver, pancreas), lung, fat cells, heart, spleen, thyroid and urinary bladder (from https://www.ncbi. nlm.nih.gov/gene).

OOEP is located in the cytoplasm.

OOEP is a component of the subcortical maternal complex (SCMC) that includes at least other three proteins, i.e. KHDC3L (also known as KH domain containing protein 3, FILIA), NLRP5 (also called Maternal Antigen That Embryo Requires, MATER) and TLE6 (also known as Transducin-Like Enhancer of Split 6). These proteins are expressed by maternal effect genes (MEGs) exclusively in oocytes and early embryos and are physically bound together in the SCMC complex. Also only a mutation on one of them, such as TLE6, induces instability of the complex and may be a cause of human female infertility and earliest human embryonic lethality (Bebbere et al., 2016; Alazami et al., 2015; Zhu et al., 2015; Bebbere et al., 2014).
OOEP plays an essential role for zygote progression beyond the first embryonic cell divisions (Bebbere et al., 2014) and it is hypotized that it could play a role in the formation/stabilization of the oocyte cytoskeleton, called oocyte cytoplasmic lattices (CPLs) and also it could be involved in the organization and regulation of the translational machinery through the interaction between SCMC complex with other protein and/or protein complexes (Bebbere et al., 2016; Tashiro et al., 2010). In addition, OOEP could be involved in RNA degradation during oocyte maturation and in the early stages of embryogenesis (Wang et al., 2012) and it could be directly or indirectly involved in the binding of the mRNAs and in their correct subcellular localization (Bebbere et al., 2016). In mouse oocytes was found that OOEP may participate in the regulation of genome stability (He et al., 2018), but it is not confirmed in humans yet.

OOEP is highly and abundant conserved in many species and its homology between the species is reported in Table.2

Organism	Species	Symbol	DNA Similarity (%)	PROT Similarity (%)
Human	H.sapiens	OOEP	100	100
Chimpanzee	P.troglodytes	OOEP	99.3	99.3
Macaco	M.mulatta	OOEP	96.0	95.3
Wolf	C.lupus	OOEP	78.7	71.8
Cattle	B.taurus	OOEP	77.4	68.6
Mouse	M.musculus	OOEP	68.2	54.6

Table.2 OOEP homology (reworked from https://www.ncbi.nlm.nih.gov/homologene?)

The genomic alterations observed include the formation of novel fusion genes as EEF1G/OOEP (acute lymphoblastic leukemia/lymphoblastic lymphoma), EXOC2/OOEP (bladder transitional cell carcinoma), FAM19A2/OOEP (breast adenocarcinoma), KHDC1/OOEP, OOEP/ EIF3A, RERE/OOEP (prostate adenocarcinoma) and SENP6/OOEP (prostate adenocarcinoma) (http://atlasgeneticsoncology.org// Bands/6q13.html), however there are no experimental data yet to understand the impact on cellular behaviour and so the implications in cancer of these fusion genes.