The function of SLC35B1 (solute carrier family 35 member B1, Ensembl gene identifier ENSG00000121073) is as follows. ATP:ADP antiporter that catalyzes the exchange of ATP and ADP across the endoplasmic reticulum (ER) membrane (PubMed:30154480, PubMed:35041824, PubMed:40399679). Imports ATP from the cytosol to the ER lumen and exports ADP in the opposite direction (PubMed:30154480, PubMed:35041824, PubMed:40399679). Regulates ER energy metabolism and protein biogenesis (PubMed:30154480). Appears to be part of a calcium- dependent ER to cytosol low energy response axis, where calcium efflux from ER to the cytosol triggers ATP import into the ER lumen to maintain sufficient ATP supply (PubMed:30154480). Provides ATP to ER chaperone HSPA5 that drives protein folding and trafficking in the ER (PubMed:30154480, PubMed:35041824). Can transport dATP, UTP or UDP in exchange for ATP, but the physiological relevance of this process remains to be established (PubMed:30154480, PubMed:35041824). {ECO:0000269|PubMed:30154480, ECO:0000269|PubMed:35041824, ECO:0000269|PubMed:40399679}. [Isoform 1]: ATP:ADP antiporter. {ECO:0000269|PubMed:30154480, ECO:0000269|PubMed:35041824, ECO:0000269|PubMed:40399679}. [Isoform 2]: ATP:ADP antiporter. .