AbstractIn this paper we describe a peculiar pattern of case alternation from the polysynthetic Circassian (West Caucasian) languages, where specificity-driven differential marking of noun phrases is attested in all syntactic positions and with the absolutive and the oblique cases alike. We call this phenomenon differential nominal marking (DNM). We show that the presenсe resp. absenсe of overt case marking in Circassian fits in the two-level (DP vs. NP) structural model for nominal constructions and is in some ways similar to the phenomenon of pseudo-incorporation described for various languages with differential object marking. For instance, unmarked nominals in Circassian show number-neutrality and scope inertness with respect to negation and quantifiers. However, DNM in Circassian crucially differs from all known instances of pseudo-incorporation or case alternation in that it is not restricted to any particular syntactic position. We argue that this feature of the Circassian DNM calls all the existing approaches (both functionalist and generative) to the phenomenon of differential case marking in question.