The DnaJ family, one of the largest chaperone families, exhibits diverse cellular localization and functions. A J domain defines membership in this family. DnaJ heat shock proteins, controlled by htpR in Escherichia coli, play a crucial role in the HSP 70 chaperone machine by stimulating ATP hydrolysis upon interaction with HSP 70. They also mediate proteolysis and regulate protein degradation, exocytosis, and endocytosis. MCJ (methylation-controlled J protein), alias HSD18, DNAJD1, or DNAJC15, is a 150-amino acid, ubiquitously expressed, single-pass membrane protein containing one J domain. Present only in vertebrates and localizing to the Golgi apparatus, MCJ may contribute to increased chemotherapeutic resistance in ovarian cancer by inducing Mdr drug transporter expression and preventing intracellular drug accumulation.