Rat Illumina and human Affymetrix microarray analyses were performed by the McGill University's Genome Quebec facility as previously described.6 (link) Briefly, RNA extracts from rat PND3 gonocytes and PND8 spermatogonia (three independent preparations for each age) were analyzed using the RatRef-12 Expression BeadChip for genome-wide expression analysis (Illumina, San Diego, CA, USA). Gene expression in the corresponding somatic cell fractions (containing 70% Sertoli cells and 30% myoid cells) were also analyzed as a comparison (two independent preparations per age). The array chip contains 22,523 probes selected primarily from the NCBI RefSeq database. Human samples (normal testicular tissue N = 3, seminoma N = 3, embryonal carcinoma N = 2, unknown pathology = 1, and Tcam−2 cells N=2) were analyzed using the HuGene-1_0-st-V1 array chip (Affymetrix, Santa Clara, CA, USA). This chip contains 33,297 probes selected mainly from the NCBI RefSeq database. Data normalization was performed by Novak et al.29 (link) Apoptosis-related genes were found by comparing already existing lists provided by Qiagen.