A kit of parts comprising two or more protein kinase substrate polypeptides, each said substrate polypeptide comprising a specificity conferring portion (which is different for each said kinase substrate polypeptide) and a phosphorylatable portion, wherein the phosphorylatable portions of each polypeptide are capable of being bound in a phosphorylation state-sensitive manner by the same specific binding partner, for example and antibody preparation, and wherein the said specific binding partner is not an antibody specific for phosphotyrosine, phosphoserine or phosphothreonine. The phosphorylatable portion preferably comprises the amino acid sequence LSFAEPG.