gene expressionbased classification of nonseminomatous male germ cell tumors male adult germ cell tumors gcts comprise two major histologic groups seminomas and nonseminomas nonseminomatous gcts nsgcts can be further divided into embryonal carcinoma ec teratoma t yolk sac tumor ys and choriocarcinoma cc on the basis of the lineage differentiation that they exhibit nsgcts frequently present as mixed tumors consisting of two or more histological subtypes often limiting correlative studies of clinical and molecular features to histology we sought to develop a molecular classifier that could predict the predominant histologic subtype within mixed nsgct tumor samples the expression profiles of 84 nsgcts 42 pure and 42 mixed and normal agematched testes were obtained using affymetrix microarrays using prediction analysis for microarrays we identified 146 transcripts that classified the histology of pure nsgcts samples with 93 accuracy when applied to mixed nsgcts the classifier predicted a histology that was consistent with one of the reported components in 93 of cases among the predictive transcripts were cgb high in cc lcn2 high in t bmp2 high in ys and pou5f1 high in ec thus the expressionbased classifier accurately assigned a single predominant histology to mixed nsgcts and identified transcripts differentially expressed between histologic components with relevance to nsgct differentiation