HSP40 (DNAJ) is an understudied family of co-chaperones. The human genome codes for over 41 members of HSP40 family that reside at distinct intracellular locations. Despite their large numbers, little is known about their physiologic roles. Recent research has revealed involvement of some of the DNAJ family members in various types of cancers. In this article we summarize the information about the involvement of human DNAJ family members in various aspects of cancer biology. Furthermore we discuss the potential role of the J domain of DNAJ proteins in cancer biology.