Ectopic Ncam1 expression in lotus tetragonolobus lectin (LTL)-negative/phosphorylated Creb (p-Creb)–positive proximal tubule–derived cyst segments in P21 Six2creFRS2α knock-out (KO) kidneys. A–D: Co-immunofluorescence for p-Creb (red) and LTL (green) or Ncam1 (red) and LTL (green) on serial sections obtained from P21 control and Six2creFrs2αKO kidneys. A: In controls, p-Creb signal is present in LTL-negative cells that likely include collecting ducts (arrows), but absent in LTL-positive proximal tubules (arrowheads). B: In Six2creFrs2αKO kidneys, p-Creb is absent in LTL-positive segments of nondilated tubules and cyst cells (arrowheads), but is expressed ectopically in LTL-negative cystic segments (concave arrowheads) and is present in many nonproximal tubule–derived cells similar to controls (arrow). C: In controls, Ncam1 staining is absent, including in LTL-positive proximal tubules (arrowhead). D: In Six2creFrs2αKO kidneys, Ncam1 expression is absent in LTL-positive nondilated tubules and LTL-positive cystic cells (arrowheads), whereas it is expressed ectopically in the LTL-negative cystic segments, overlapping with p-Creb (concave arrowheads) (compare B and D). Scale bars: 50 μm.