Tüttelmann F, Werny F, Cooper TG, Kliesch S, Simoni M, Nieschlag E. Clinical experience with azoospermia: aetiology and chances for spermatozoa detection upon biopsy. Int J Androl. 2011 Aug;34(4):291–8. doi: 10.1111/j.1365-2605.2010.01087.x. PMID: 20609027.