x assignee

Al menos en México, así es.

“herederos y causahabientes” es una expresión casi inevitable en materia de sucesiones.

Saludos,
Manuel
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causahabiente – persona que adquiere o que tiene derecho a adquirir de otra (llamada autor o causante) o una obligación. Los causahabientes son llamados también derechohabientes. Ambas expresiones se aplican especialmente en materia de sucesiones, y así, el heredero es el causahabiente o derechohabiente del que deja la herencia, que es causante o autor.
[Ossorio, Manuel: Diccionario de Ciencias Jurídicas, Políticas y Sociales, Editorial Heliasta, 27ª Ed., ISBN 950-885-005-1]
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assign, assignee – Both words mean “one to whom property rights are transferred by another”. Assignee is more understandable to nonlawyers, who know assign as a verb only. The doublet “heirs and assigns” is unlikely to dissappear, however; assign as a noun almost always appears, as in the phrase just adduced, in the plural.
[Bryan A. Garner: A Dictionary of Modern Legal Usage, Oxford University Press, 1995 – ISBN 0-19-507769-5]
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ASSIGNS, contracts. Those to whom rights have been transmitted by particular title, such as sale, gift, legacy, transfer, or cession. Vide Ham. Paities, 230; Lofft. 316. These words, and also the word forever, are commonly added to the word heirs in deeds conveying a fee simple, heirs and assigns forever "but they are in such cases inoperative. 2 Barton's Elem. Convey. 7, (n.) But see Fleta, lib. 3, cap. 14, 6. The use of naming them, is explained in Spencer's Case, 5 Rep. 16; and Ham. Parties, 128. The word heirs, however, does not include or imply assigns. 1 Anderson's Rep. 299.
http://www.constitution.org/bouv/bouvier_a.htm