The PCNA gene is situated on human chromosome 20 and it spans about 12 kb. It is a single-copy gene, however, several pseudogenes have been noted. The precise localization of the PCNA gene is at the border of two histological G-bands (p12.3 and p13) (Webb et al., 1990), thus it is reported in both locations depending on the probe used. The human PCNA gene was first cloned and characterized in 1989 by Travali and co-workers (Travali et al., 1989).

There are two reported gene transcripts, which encode the same protein.

	NCBI Reference Sequence	Length (unspliced)	Length (spliced)	Exons	Protein	AA
PCNA transcript variant 1	NM_002592.2	11670 bp	1355 bp	7	NP_002583.1	261
PCNA transcript variant 2	NM_182649.1	5049 bp	1319 bp	6	NP_872590.1	261

PCNA transcript variant 1 is 1355 bp long after the completion of mRNA splicing. It has NCBI Reference Sequence code NM_002592.2 (NCBI). The PCNA transcript variant 1 has seven exons, six of which are contributing to the protein sequence. The first intron is relatively large in comparison with the other PCNA transcript variant. Following the splicing the length of the transcript is shortened to about 12% of that of the initial transcript. The translation starts from the middle of the 2^nd exon and ends in the beginning of 7^th exon. The product is a full length protein, designated as NP_002583.1 (NCBI), with 261 amino acids.
PCNA transcript variant 2 is 1319 bp long after the completion of mRNA splicing. It has NCBI Reference Sequence code NM_182649.1 (NCBI). The PCNA transcript variant 2 has six exons, which are contributing to the protein sequence. After the splicing the length of the transcript is shortened to about 26% of that of the initial transcript. Translation starts from the end of the 1^st exon and ends in the beginning of 7^th exon. The product is a full length protein, designated as NP_872590.1 (NCBI), with 261 amino acids.

PCNAP - one pseudogene on human chromosome X - p11 (Ku et al., 1989; Webb et al., 1990).
PCNAP1 and PCNAP2 - two pseudogenes in tandem on human chromosome 4 - q24 (Taniguchi et al., 1996).
There are several other possible pseudogenes:
LOC390102 on chromosome 11 - p15.1 (Webb et al., 1990),
LOC392454 on chromosome X - p11.3 (Ku et al., 1989; Webb et al., 1990).

The human PCNA protein is a polypeptide of 261 amino acids and theoretical molecular weight of about 29 kDa. The functional protein is a homotrimer, build from three identical units interacting head-to-tail and forming a doughnut shaped molecule. There is an evidence for the existence of a double homotrimer in vivo (Naryzhny et al., 2005).

Expressed in nearly all proliferating tissues with high levels detected in thymus, bone marrow, foetal liver and certain cells of the small intestine and colon.

PCNA is exclusively localized in the nucleus. It can be detected by immunofluorescence in all proliferating nuclei as discrete nuclear foci, representing sites of ongoing DNA replication and/or DNA repair.

PCNA was originally discovered as an antigen, reacting with antibodies derived from sera of patients with systemic lupus erythematosus (Miyachi et al., 1978). The first assigned function of the PCNA protein is as an auxiliary factor of polymerase delta (Tan et al., 1986; Prelich et al., 1987). Later it was suggested that PCNA functions as a cofactor to many other eukaryotic polymerases such as polymerase epsilon, polymerase beta and several specialised polymerases known as translesion synthesis polymerases (eta, kappa, lambda, theta, etc.), with which PCNA is known to interact (Naryzhny, 2008). The role of PCNA in DNA replication is thoroughly investigated and PCNA is proposed to serve as a switch between the priming polymerase alpha and replicative polymerases (delta and epsilon) and functioning as a cofator of the latter polymerases. Complementary to enhancing the processivity of DNA replication, PCNA is known to coordinate the maturation of Okazaki fragments through interaction with FEN1 and stimulation of the flap endonuclease activity. PCNA interacts with large number of proteins, suggesting many functions in vivo (Naryzhny, 2008; Stoimenov and Helleday, 2009). There is evidence, derived from experiments in yeast, that PCNA may be involved in the establishment of sister chromatid cohesion in S phase of the cell cycle (Moldovan et al., 2006). PCNA is an indispensable factor for different DNA repair pathways including mismatch repair, nucleotide excision repair and sub-pathways of base excision repair. There is a growing body of evidence for the function of PCNA in the chromatin remodelling and organisation. The interaction of PCNA and CAF1 is in the heart of the nucleosome assembly, while the chromatin modification is also known to be regulated by PCNA through the known interaction with DNMT1 and HDAC1. One of the most stable interactions of PCNA is that with the cyclin-kinase inhibitor CDKN1A, which suggests a role of PCNA in the cell cycle progression. Another evidence for the involvement of PCNA in the cell cylcle control is the interaction with cyclin-D. Several amino-acid residues are post-translationally modified, suggesting even more complex functions (Stoimenov and Helleday, 2009). PCNA could be subjected to post-translational phosphorylation, acetylation, methylation, ubiquitylation and SUMOylation.