The marsupial conceptus is surrounded by a uterine-secreted shell coat for 60-80% of gestation. Coat protein 4 (CP4) is the only marsupial shell coat protein characterized and it has only been identified in one species, the Common Brushtail Possum. In this possum, uterine transcription and secretion of cp4 during the oestrous cycle is biphasic and associated with the stage of conceptus development. Here we cloned cp4 (sm-cp4) from a distantly related species, the Stripe-faced Dunnart (Sminthopsis macroura). Transcription of sm-cp4 and secretion of smCP4 were identified by semi-quantitative RT-PCR and immunohistochemistry, respectively. The effect of reproductive hormones on sm-cp4 transcription was investigated in vitro by treatment of uterine explant cultures with oestrogen and/or progesterone. In vivo, uterine expression of smCP4 was biphasic and associated with conceptus development. Uterine smCP4 expression (transcription and secretion) began during the pre-ovulatory period and continued post-ovulation during cleavage stages. Transcription of smcp4 continued during the unilaminar blastocyst stage, but smCP4 secretion was reduced during this stage. During the bilaminar blastocyst stage, both transcription of sm-cp4 and secretion of smCP4 were low before they both resumed during the trilaminar blastocyst stage, and continued during the embryo and fetal stages. In vitro uterine transcription of sm-cp4 increased after incubation with progesterone.