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During the third week of pregnancy, the process of gastrulation results in the formation of trilaminar embryo in humans. This is the beginning of morphogenesis (development of body form). Three germ layers, ectoderm, mesoderm, and endoderm, are present. A specific part of mesoderm, the intermediate mesoderm, gives rise to urogenital system gradually. It starts to develop from nephrogenic cord, which divides in nephrotomes. They give rise to pronephroi and subsequently mesonephroi, the transitory kidneys. Finally, metanephroi appear as the permanent organs, from two different structures: the metanephrogenic blastema (mesenchimal component) that leads to nephrons, and ureteric bud (epithelial component) that gives rise to collecting tubules, calices, renal pelvis, and ureter. Following developmental steps depend on inductive signaling between metanephrogenic blastema and ureteric bud. Many genes, regulating proteins and pathways are involved in the physiological organogenesis. Cell proliferation and apoptosis keep the balance of the growth. Defects in these molecular and morphogenetic mechanisms may cause various congenital abnormalities of the kidney and urinary tract (CAKUT), which represent a family of diseases with a diverse anatomical spectrum.

The supernumerary kidney is an accessory organ with its own blood supply and collecting system. It is a very rare type of congenital renal anomaly with fewer than 100 cases reported since firstly described at 1656 (1). Embryological basis of supernumerary kidney is connected to the ab-normal division of the nephrogenic cord into two metanephric blastemas which will form two kid-neys (2). It may be either completely separate or only loosely attached to the major kidney on the ipsilateral side. This anomaly is usually asymp-tomatic but may rarely become symptomatic in early adulthood (3). The mean age at diagnosis is 36 years. The most common presenting symptoms are pain, fever and a palpable abdominal mass. Ultrasonography, CT and MR urography may be needed to identify the anomaly.We aimed to present a rare case of super-numerary fused and malrotated kidney. Thirty--seven year-old woman was admitted to our clinic with left flank pain. CT scan demonstrated multi-ple calculi in the left kidney and supernumerary fused kidney (Figures 1 and 2). The third kidney was below the right kidney, malrotated and had its own blood supply (Figure-3).Grieshammer and colleagues showed that mutant mice lacking either SLIT2 or its receptor ROBO2 develop supernumerary ureteric buds that are correlated with abnormal maintenance of Gdnf expression in anterior metanephric mesenchyme (4). The SLIT2/ROBO2 intercellular signaling sys-tem restricts, directly or indirectly, in extent of the Gdnf expression and plays a critical role in