“…On the other hand, it is difficult to understand the expression of both Rag and/or Ig chains before the appearance of thymus and pronephros. Some authors have remarked the high frequency of sterile and aberrant transcripts that occur in teleosts [19] but, on the other hand, evidence of TcR/BcR gene rearrangements before lymphoid progenitors colonize the primary lymphoid organs has been found in mammals [38,39]. The expression of Hox-11 transcripts in a small region on the left anterior gut marks the appearance of the zebrafish splenic primordium by day 4 postfertilization [40].…”