During meiosis, homologous chromosomes pair to form bivalents, enabling crossing-over—the physical exchange of chromatid segments. This crucial step requires proper alignment and synapsis of chromosomal pairs, a task mediated by the synaptonemal complex (SC). The proteins involved in homologous pairing and SC formation are beginning to be identified, spurred by the fact that defects in genome replication are hallmarks of severe genetic disorders, such as cancer and Down syndrome. In the May 23 Science, Michael Crackower and colleagues at the Institute of Molecular Biology of the Austrian Academy of Sciences report that a newly identified protein component of the SC, Fkbp6, is also critical for male fertility in mice and rats (Science, 300:1291-1295, May 23, 2003).

Crackower et al. used immunochemical techniques to localize Fkbp6 in mice and detected protein expression within the SC of spermatocytes, associated with meiotic chromosome cores and regions of homologous chromosome synapsis....

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