The function of RCC1 (regulator of chromosome condensation 1, Ensembl gene identifier ENSG00000180198) is as follows. Guanine-nucleotide releasing factor that promotes the exchange of Ran-bound GDP by GTP, and thereby plays an important role in RAN-mediated functions in nuclear import and mitosis (PubMed:11336674, PubMed:17435751, PubMed:1944575, PubMed:20668449, PubMed:22215983, PubMed:29042532). Contributes to the generation of high levels of chromosome-associated, GTP-bound RAN, which is important for mitotic spindle assembly and normal progress through mitosis (PubMed:12194828, PubMed:17435751, PubMed:22215983). Via its role in maintaining high levels of GTP-bound RAN in the nucleus, contributes to the release of cargo proteins from importins after nuclear import (PubMed:22215983). Involved in the regulation of onset of chromosome condensation in the S phase (PubMed:3678831). Binds both to the nucleosomes and double-stranded DNA (PubMed:17435751, PubMed:18762580). {ECO:0000269|PubMed:11336674, ECO:0000269|PubMed:12194828, ECO:0000269|PubMed:17435751, ECO:0000269|PubMed:18762580, ECO:0000269|PubMed:1944575, ECO:0000269|PubMed:20668449, ECO:0000269|PubMed:22215983, ECO:0000269|PubMed:29042532, ECO:0000269|PubMed:3678831}.