DNA sequence containing a pros-RJ cDNA with mutations in the N7 motif that impair its liquid-liquid phase separation activity (amino acid replacements M828S, L838S, I846S, F861S and F868S), along with 2039bp of sequence upstream of the pros-RJ ATG start codon (including pros intron, promoter and 5'UTR) and 3265bp pros 3'UTR, and with the coding sequence tagged with Tag:MYC, has been introduced into the attP site present in TI{TI}prosKO.