In some cases, anti-ubiquitin immunoreactivity is seen in the region of stroma underlying the luminal epithelium of nonpregnant women (Fig. 1a, labeled ā€˜ā€˜pā€™ā€™). Interestingly, these tissues were taken from women in the late secretory phase of the menstrual cycle and may represent cells undergoing pseudo-decidual-ization; together with the observations of immunoreactivity in pregnant tissue (Fig. 1, c, e, and f), the results indicate that decidualization is accompanied by an increase in anti-ubiquitin immunoreactivity. The pattern of anti-ubiquitin-immunoreactive polypeptides after electrophoresis and im-munoblotting (Fig. 2) of nonpregnant and pregnant human endometrium probably represents the presence of many different polypeptides conjugated to multi-ubiquitin chains of different sizes.

The absence of an immunoreactive band corresponding to ubiquitin monomer may simply reflect the lack of sensitivity of this antibody to the free form of ubiq-uitin and the relatively low levels of free ubiquitin in the tissue. Anti-ubiquitin immunoreactivity may be the result of the presence of ubiquitin, UCRP, or ubiquitin and UCRP together. Human decidual cells are immunoreactive to the specific anti-UCRP antibody (Fig. 3), indicating that at least some of the anti-ubiquitin immunoreactivity seen in sections of human tissue containing decidual cells is due to the presence of UCRP.