Three infertile subjects with no expression of CD46 (membrane cofactor protein of complement) on their spermatozoa were found when screening 542 idiopathic male infertile patients. The sperm CD46 isoform was reported to be associated with the sperm-egg interaction, yet a ubiquitous expression of CD46 confers resistance to complement-mediated injury on host cells. All three patients expressed normal CD46 isoforms on their lymphocytes and granulocytes. Thus, the loss of CD46 is sperm-specific, probably due to testicular germ cell-specific regulation of CD46 production. Recently, a mechanism of the gene regulation of human CD46 was elucidated in which the silencer element of the 3′ UT and the promoter region of human CD46 gene partly participate. Here, we analyzed these regions of the CD46 gene in our 3 patients. We found no abnormality in 3′ and 5′ regions of the CD46 genome in the 3 patients. Thus, in these infertile patients sperm-specific depletion of CD46 is not governed by the so for identified regulators in the CD46 gene. Other unknown factors outside the known regulatory regions would play a role in the regulation of sperm-specific CD46 expression.