In the Indo-European domain, the grammatical category of gender has undergone a manifold evolution, leading, in some languages, to the complete neutralisation of gender distinctions. More commonly, the three-gender system of older non-Anatolian Indo-European languages has been reduced to a twofold category, where the neuter gender was often lost and the void was filled by the masculine and the feminine. While the modalities in which gender is expressed can be typologically complex, there is little debate concerning the number of gender values in most Indo-European languages. For a few others, however, the matter is more intricate. Tocharian and Khotanese, two Indo-European languages formerly spoken and written in the Tarim Basin (present day Xinjiang Uygur Autonomous Region of China), exemplify such complexities. This presentation thus aims to examine the gender systems of these languages, focusing on the ambiguous status of the so-called genus alternans, i.e. an agreement class characterised by an unusual combination of forms available for agreement with nouns in the other gender values of feminine and masculine.