An antibody or antigen-binding portion thereof which binds to PSMA and comprises a heavy chain variable domain comprising the sequences: CDR1: EYTIH (SEQ ID NO: 33) CDR2: NINPNX1GGTTYNQKFED (SEQ ID NO: 34) CDR3: X2-5DY (SEQ ID NO: 35) wherein X1 is N or Q, and X2-5 is YWLF (SEQ ID NO: 39), GWTF (SEQ ID NO: 40) or AWTM (SEQ ID NO: 41), and wherein if X2-5 is GWTF (SEQ ID NO: 40) or AWTM (SEQ ID NO: 41), the amino acid residue at position H94 in the heavy chain variable region, based on Kabat numbering, is G; and if X2-5 is YWLF (SEQ ID NO: 39), the amino acid residue at position H94 in the heavy chain variable region, based on Kabat numbering, is A. The invention also provides compounds that include the antibody or antigen-binding portion thereof, such as conjugates, and their use in the treatment or diagnosis of disease, in particular cancers, particularly prostate cancer.