Homeobox gene HB24, a regulator of haematopoiesis, is a candidate for regulating differentiation of the extra-embryonic trophoblast cell lineage.

Expression of the human homeobox gene HB24 in the cytotrophoblast stem cell population in first-trimester human placenta was investigated. HB24 expression was downregulated after cytotrophoblast had differentiated into syncytiotrophoblast. Expression of HB24 was also detected in first-trimester invasive extravillous cytotrophoblast cells. In term placenta, HB24 expression was detected in residual cytotrophoblast cells and in syncytiotrophoblast. Northern analysis revealed two HB24 transcripts in first-trimester placenta of approximately 2.5 kb and 5.3 kb. Ribonuclease protection assays were used to confirm expression of HB24 in first-trimester and term placenta and also to provide evidence that HB24 is downregulated in choriocarcinoma cell lines. In addition to being required for differentiation of the haematopoietic cell lineage, HB24 may be required for differentiation of the extra-embryonic trophoblast cell lineage.