Mucins secreted by the endocervical epithelium protect the surfaces of the reproductive tract epithelium from pathogen penetrance and modulate sperm entry into the uterus. Three large gel-forming mucins, MUCs 5AC, 5B, and 6, are expressed by the endocervical epithelium, as is MUC4, a relatively uncharacterized mucin for which only tandem repeat sequence has been reported. We sought to determine the relative abundance of each of these mucin gene transcripts and to relate their expression to blood progesterone and estradiol. Samples were obtained from six subjects at successive stages in the menstrual cycle. Primers to nontandem repeat sequences of MUCs 4, 5AC, 5B, and 6 were used in semiquantitative reverse transcription-polymerase chain reaction to determine relative abundance of each mucin gene in relation to beta2-microglobulin message control. In order to design primers from a nontandem repeat region of MUC4 so that MUC4 message levels could be quantitated, we obtained approximately 2.7-kilobase nontandem repeat sequence 5' to the tandem repeat sequence of a MUC4 genomic clone. The sequence showed lack of cysteine-rich D-domains and was rich in serine and threonine. Semiquantitative polymerase chain reaction analyses indicated that the principal mucin transcripts of human endocervix are MUC4 and MUC5B, with MUC4 predominant in 15 of 21 samples. When correlated with plasma steroid levels, message levels of both MUC4 and MUC5B were inversely related to progesterone levels.