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Abstract: We present deep observations taken with the HST advanced camera for surveys of the central massive galaxy in a forming cluster at z =2.2. The galaxy hosting the powerful radio source MRC 1138–262 is associated with one of the most extensive merger systems known in the early universe. Our HST/ACS image shows many star-forming galaxies merging within a ∼200kpc region that emits both diffuse line emission and continuum in the rest-frame UV. Because this galaxy lives in an overdense environment, it represents a rare view of a brightest cluster galaxy in formation at z >2 which may serve as a testbed for predictions of massive cluster galaxy formation. [Copyright &y& Elsevier]