Despite its ambiguous name the protein melanoma-inhibitory-activity (MIA) was identified as a key molecule involved in progression and metastasis of malignant melanomas. Therefore, in this review we intend to update the current knowledge on expression patterns, transcriptional regulation, function and clinical relevance of MIA. Furthermore, we will cover the recently discovered MIA homologous proteins OTOR/MIAL, MIA 2 and TANGO. In order to identify autocrine growth-regulatory factors secreted by melanoma cells, MIA was purified and cloned. Subsequent analyses of non-neoplastic tissues revealed specific MIA expression patterns in cartilage. In neoplastic tissues MIA expression was detected in malignant melanomas, in chondrosarcomas and less frequently in a variety of different adenocarcinomas including breast and colon cancers. For melanoma cells and chondrocytes it was shown that regulation of expression pattern was controlled on the level of mRNA transcription by defined transcription factors. Evidence obtained from in vitro and in vivo experiments indicated that MIA plays an important functional role in melanoma metastasis and invasion. A number of studies from different laboratories evaluated MIA as a highly specific and sensitive marker, clinically useful for follow-up and therapy-monitoring of patients with malignant melanomas. In addition, preliminary data suggests a further potential application as a surrogate marker for measuring cartilage damage in rheumatoid arthritis. Recently, it has become evident that MIA belongs to a gene family of four homologous proteins, MIA, OTOR (FDP, MIAL), MIA 2 and TANGO. Determination of the three-dimensional structure in solution identified MIA as the first member of this novel family of secreted, extracellular proteins adopting an SH3 domain-like fold. The data suggest specific protein-protein interactions with components of the extracellular matrix and possibly epitopes on cellular surfaces and will certainly attract further interest and investigations.