MAT1A and pneumocystosis: Three of our observations are strongly suggestive of primary homothallism in Pneumocystis species, (i) the presence of the same MAT locus in all P. jirovecii isolates investigated, (ii) the lack of other MAT transcription factors in the genomes and of any sequences used for switching or silencing, and (iii) the frequent concomitant expression of the three MAT genes during PCP.