A homokaryotic strain (5337) in our culture stock of Coprinus cinereus produced fertile fruit bodies after prolonged culture. Microscopic examination revealed that hyphae dedifferentiated from the tissues of one of the fruit bodies, as well as all basidiospore derivatives from the fruit body, exhibited pseudoclamps, whereas vegetative hyphae of 5337, from which the fruit body developed, had no clamp connections. Genetic analysis showed that the formation of pseudoclamps results from a recessive mutation in a gene designated pcc1 (pseudoclamp connection formation), which is distinct from the A and B mating type genes. Cloning and sequencing of the pcc1 gene and cDNA identified an ORF of 1683 bp interrupted by one intron. Database searches revealed that pcc1 encodes an SRY-type HMG protein. The HMG box shared 44, 41, and 29% sequence identities (>80 amino acids) to those of FPR1 of Podospora anserina, MAT-Mc of Schizosaccharomyces pombe, and prf1 of Ustilago maydis, respectively. Northern analysis revealed that the level of pcc1 expression is higher in the dikaryon, in homokaryons in which the A and B mating type developmental sequences are individually activated, than in the homokaryon in which these sequences are not active. Sequencing of the pcc1-1 mutant allele revealed that the mutant carries a nonsense mutation at serine 211, a residue located between the HMG box and the C terminus. Based on these results, possible roles of the pcc1 gene in the sexual development of homobasidiomycetes are discussed.